Usually an informal trip just a group climbing trip with no set skills being taught. Instead the idea is just to do some climbing, and meet other climbers in the process. The assumption is that the participants already know the basic safety and anchor skills. They are instead working more on their climbing techniques and fitness, and meeting some other climbers for potential future trips (these things can be important for safety as well, once you have learned the basic technical skills).
There will usually be some more experienced climbers on an informal trip, and they will tend to look after the less experienced folks, but in general the participants will have to assess the safety and skill levels of the other people themselves. So we advise that you know your partners (from previous classes/trips, gym climbing, mutual friends, etc.), and that you inspect and test the anchors.