If you like to write, join the magazine or newspaper club. If you like acting, show your talent in the school play. Sport teams are great too. Invite friends over to your house or to a movie, as this may help tie your friendship together. Ask your parents to invite a few classmates and their parents over for a pool-party or a barbarcue.
Some groups revolve around on person.
The person in the center can get very stressed because the other members watch their every movement. Often, when the central person makes a mistake, the surrounding group of admirers back off, seeing they aren't perfect. Then the group either falls apart or elects a new center. There are also smaller, friendlier groups where everyone is treated equally. These are the groups you should join.