What does everyone actually wear to one of these?

Almost embarrassed to ask this. There is a daytime social in a fortnight and I have been going back and forth about what to wear, which is not a sentence I have ever said about a coffee.

The specific worry is that this is a group of people who are here for one particular reason, so I do not know whether there is an unspoken thing about footwear. Do people wear sandals? Would that be presumptuous? Would wearing boots look like I was making a point?

I suspect the answer is that everyone wears normal clothes and nobody has ever thought about this except me. Please confirm so I can stop.