If you can practice riding in a group before hand, that might make you more comfortable on the event day. Give yourself enough space when you ride so that you have time/room to maneuver if there is a dropped bottle or someone ahead of you makes an unpredictable move.

My understanding is that sportives are not competitive, so I'm guessing you won't have to worry about staying out of the way of faster riders at the start- but position yourself further back if that might be a problem. How many riders will there be?

33 miles is quite a reasonable distance to manage- you should have a wonderful time.