There's a couple things that could be going on, and if your friends aren't forthcoming on questions beforehand, you won't know until you get there.
That said, my first suggestion is that if you have a really bad feeling about this, don't go. You don't even have to make up an excuse. Tell them that if you don't have at least some idea what this is about, you aren't going, period, end of story.
Here’s the advice, simple and unadulterated: Just tell the truth.
Not some version of the truth, or part of the truth, or truth that’s technically true but isn’t the real reason for your behavior. It’s got to be, as they say, “the whole truth."
You don't have to stoop to the same shadow tactics they are, "Oh, I've got something else to do that day", since that'll likely just end up in a reschedule, if they are insistent. This isn't about conflict avoidance at this point, it's about your safety and well being, including your mental well being as well as your physical well being.
If you do go and it's a public area, make sure you are able to get out of the situation on your own. If it's a restaurant, have them both sit on the opposite side as you so you can get out without asking one of them to move. If something shady is going on, you'll have an easy and possibly quick egress.
SIT CLOSE TO THE AISLE
Whether choosing a seat at a table, a movie theater, or anywhere with a row of seats, sit close to the aisle. This will allow you to get out quickly and less likely to be become slowed during an emergency situation.
DON’T BOX YOURSELF IN
If sitting at a restaurant table with a bench style seat or U-shaped seat, sit on the end. When the group is being seated simply allow others to slide in before you and take up the end…
And if something really bad happens, don't be afraid to "make a scene". If they hurt you, be loud. If you've seen the movie "Hitch" with Will Smith, you might remember when Will meets with the obnoxious exec with the power suit, power ties, power moves etc in the restaurant. When Will get uncomfortable, he leaves. When the SOB grabs Will's arm, Will give him a brief warning to let go. When he doesn't, Will very publicly face-plants the guy on the table. Of course, this is a "last resort" kind of thing, but you don't always have to be nice to people, especially when they aren't being nice to you, and extra especially when they are endangering you.
If You Are Kidnapped
The initial phase of a kidnapping provides the best opportunities to escape.
If you are in a public area at the time of an abduction attempt, make as much commotion as possible to draw attention to your situation.
If the abduction attempt takes place in your hotel room, make noise, attempt to arouse the suspicion or concern of hotel employees or of those in neighboring rooms.
Ok, so on to better scenarios.
This meeting could be benign. These two people might just be getting bored with the rest of the group and don't want to admit it in front of the rest of the group. They might want to start a new group, believing you are interesting enough to join it. This kind thing happens, where some people outgrow others or realize they can't stand someone anymore. If you still want to hang out with the people left out of the new group, then that's something you'll have to figure out: do you want to do something different with these "leftovers" from the original group or are you even interested in the new group.
Fortunately, most friendships have a natural life cycle. Forti says that leaving a certain friend or group usually comes along with a shift in identity (your kids have grown, you no longer run as often, etc.). And when you naturally outgrow these groups, the intensity of the relationships tends to diminish and the parting of ways can happen on good terms.
This meeting could also be good, something the two people have been wanting to collaborate with you on and don't need to include the rest of your group, such as a new business. They realize you can do something better/faster than they can, maybe they just need more hands on deck, maybe they need someone to tell them an "outsiders" view of the thing, and they trust you. Maybe it's a surprise party they are planning for the other two.
Or the meeting could be something helpful that you don't want to talk about.
Dude, you drink so many Pepsi/Coke/Mt. Dew/Dr. Peppers during our meetings that we think you're going to float away in all the liquids. You need to slow down before you end up weighing 1000 lbs.
I have no way of knowing what this could be, but it seems like a possibility. Whether it's a bad habit of yours, a tragedy you are still suffering from, a big decision you've made they disagree with, or maybe a decision the group made they don't agree with. Maybe they think you are too shy and need to be more outgoing, or simply "need to get out of the house more often". Maybe they want you as a "wing man", or they want to be your "wing man". Maybe they've met someone that would be "perfect" for you and want to introduce you to them, or maybe they want to include you in their relationship.
Even if they do agree to the date, you should still back off, Spira says. As a matchmaker, it's your job to introduce the idea of going on a date and to exchange contact information or introduce them in person. It's not your job to plan the date for them — unless you're also going to be there. "The easiest solution is to suggest a double date or group date to introduce someone new to your friend," Spira says. "This removes the pressure of having them be alone with someone they don’t end up having chemistry with, which can backfire and be dreadful." But if they'd rather get to know each other privately, then pass along contact info and let them take it from there.
There are vastly more possibilities than I gave examples for, and this was not supposed to be an exhaustive list, but I think this covers the main categories of possibilities of what could happen.
And for asking more questions about the meeting, are you doing this in person so you can gauge their reactions? Is it a shy, defensive, or offhand denial of information. They could just be embarrassed about the topic or they could think you are embarrassed about it. If forceful about not answering your question, they might be doing something less than legit, which would likely mean you don't want to go to this meeting. And if it's an offhand comment about it being nothing to worry about, they still haven't given you enough info to make a decision on, since it could be just a conversation or they could be lying.
You already pick up on more body language cues than you're consciously aware of. UCLA research has shown that only 7% of communication is based on the actual words we say. As for the rest, 38% comes from tone of voice and the remaining 55% comes from body language. Learning how to become aware of and to interpret that 55% can give you a leg up with other people.
Without knowing what's going on, your anxiety is understandable. You can choose to let it freeze you out of going, or you can go in spite of your misgivings. Really, only you can answer this question. Again, you won't know until you either ask for more info beforehand or you get there. And again, if you don't feel comfortable with the meeting: don't go and don't mince words about it either.
To address a comment, a meeting to discuss a business partnership isn't a worst case scenario. Even if it's a scam/MLM/pyramid scheme/whatever, it's still something you can walk away from without any lasting harm. A physical assault or kidnapping are far worse scenarios.
Also, can you imagine what some friends of Mark Zukerberg are thinking now, if they had denied meeting with him about this crazy idea of a social networking platform he wanted to build just because it was a business deal/partnership offering? Or Steve Jobs, Bill Gates, or any of the large companies that exist today? Yes, there's plenty of failed businesses around, and many that are struggling, but as long as a person knows when to get out, before they lose the shirt off their back, a meeting about a startup business isn't a worst case scenario.
Launching a business isn’t easy, which is why having a co-founder can often make things a little smoother, especially if that co-founder is your best friend. While many professionals warn against going into business with close friends, there are plenty of examples that prove it can work. Just take a look at companies such as Airbnb, Warby Parker and even Ben and Jerry’s. These successful businesses started out with a friendship before a brand, and look how they turned out.