Check out each topic in the group build section,see what the parameters are for each build, like subject,timeframe,time frame, rules set by group leader.If it fits,then sign up,either on that thread or by messaging the leader.You are usually required to post a dated start picture showing your subject unbuilt, you are usually required to post at least three progress shots.Finally you need to post pictures of the finished model before the due date.
You should expect commendation and constructive criticism of your build,and you should definitly be checking in and commending or encouraging your fellow builders on the thread.Thats what a group build is,several members building the same theme supporting and encouraging each other.
If you are successful, the group leader will award a badge that you can post in your profile.See the handful that I have on the bottom of my posts.
If handled correctly,they are fun,don't join and not participate or just drop out,when too many do that it ruins the fun for the others.Don't be self centered,pay attention to others work.
Usually you can get in on the ground floor when someone posts an interest post to see if anyone is interested in starting a new one.