I think there are three key things to find out:
1. Are there any kit or clothing restrictions? TT bars are often banned, a certain grouping often requires certain headgear, some require certain types of lights and I've heard of one that asks people not to wear bibs as they're not waiting that long at the coffee stop!
2. Is it no-drop? If there's a risk of getting dropped, you need to know how to get home and the kit to do so, be that cleat covers in case you're walking, or a phone to call a taxibus.
3. Is it freewheeling? If not, pedal on the downhills, especially if you find yourself on the front of a bunch, as the slipstream effect means those behind will be catching you up and having to brake. If it is freewheeling, either spread out as you cross summits or be ready to brake on downhill straights.
As others said, introduce yourself as new at the start (get there a bit early!) and it should go smoothly. I like group riding and my local is everything-welcome no-drop freewheeling.