The invite form said we could bring a guest.
Then, attending the event with a plus one is fine. It's basic etiquette because you're just following the house rules for this meeting. There are many (un)written rules when it comes to official gatherings, like weddings. But informal and friendly meetings are mostly based on education more than on strict protocol. You behave like friends do, following basic educational and social rules. Politness first and (almost) only.
I have a close friend who I'm considering inviting to go with me. However, I'm concerned that people might think this person is my girlfriend.
She will be who she is. If she's your friend, she'll behave like one. She'll be able to tackle most personal questions about "who she is" (read: GF / SO) or "what's your relationship". Then, things will be cristal clear, even though you guys may have to repeat again and again that you're just friends.
I have some social awkwardness and didn't really form my friend group until after high school.
Time to meet some "new" people and improve your social skills. It's a great opportunity to talk about the "good ol' times". I've been to such parties, and despite having an excellent memory, I couldn't possibily remember all the folks there. You have fun playing the "who-do-you-think-I-am?" game. Be straight, be honest, give and take clues, just be who you are.
Basically, you talk and listen about the past 10/15/20 years. You can just talk about "before" or "after" we left school, or "since we were there". If either one of you is stuck, politely move on from one place/group to another once in a while, or go to the buffet (if any), to the bar...
I like that guide to plus-one etiquette for parties by Jen Kirsch, because it shows many different aspects of these meetings, and the way to handle them. It's quoted below.
So what is proper protocol when you’re invited to an event with a guest?
Natasha Koifman says that for any event, you have to think about your audience, including the host and the other guests in attendance. If you are single or your partner isn't available your plus one should be someone you genuinely have a good time with but who also possesses good social graces.
It's important to follow your gut and be aware of who makes you shine versus who dims your light. You don't want to be babysitting someone all night or struggling to include them in conversation.