I had once a similar situation, and wanted to tactfully test the waters. Once I heard of the event, I waited for a little over a week and a good opportunity at the coffee machine.
I spotted that colleague, alone1 at the moment, and started small talk. Then, I just said:
I heard you guys were going to Central Perk for a drink on Friday. I was thinking about going with my SO one time maybe. How is it? Do you know it?
You ask some trivial questions about the place, not about their party. Don't show interest in their party, show interest in the place hosting it. It's less intrusive. Doing this, you let them know that you know about their party, without asking for anything else or going any further. Now, the ball is in their court, it's their move.
If the colleague picked up the hint (you knowing about the event, and you asking about the place), he might connect you and the event, and later talk to the others chaps. That's what you wanted to do: link your name, the place and the event in his mind.
From their response, you'll have more insight about what they intend to do. Are they elusive? Are they deflecting? Are they denying or downsizing the fact that they're going? Any of these, I always understood it as "forget about it". Until/unless you have some (other) words to interpret, I see no other way to smoothly and with delicacy test the waters.
1. I would not recommend doing that in the middle of the group because they might not be able to answer, it could make them feel uncomfortable, as no one would know what the others think.