oh its a sportive, so maybe it won't be so horrible. Are they less competitive or something?
They're not technically races – they are times "challenges". While some may treat it as a proper race, others will just ride it as a regular ride.
My experience of sportives is that people tend to ride together then split up, and so you don't need to worry so much about group riding in the same way. If you don't feel confident riding in a group, don't. Equally, if you think it's OK, then you'll need to do your fellow group members certain courtesies (for safety purposes): (1) keep a steady line – don't swing around the road (2) before changing line, have a quick look over your shoulder, (3) point out obstacles, holes etc. in the road, (4) warn riders ahead of a car behind by saying "car up", and warn riders behind of a car ahead by saying "car down". Do all these things, and they'll think you a seasoned pro.
Most of all, have fun. It's not very far, and doesn't have too much climbing, so have a nice day out!