Make sure your group members feel appreciated and valued.   Let them know that you respect what they contribute to the group dynamic and that you wouldn't be where you are as a group without them. Everyone needs to feel like they are an important part of the group. Make sure everyone is kept informed of plans, gigs, and all events. Don't leave anyone in the group out of the loop. Avoid gossip. Don't spread rumors about other members of your group. It's easy to get caught up in the business aspects of making and practicing music together.   Make sure that you make time to do some fun things together as a group. Since you spend so much time together, it's important that you work on your friendship as much as you work on your vocals. Take some time to go to a party together or spend a day at an amusement park. Whatever you do, just make sure you're having fun with your group members. Participating in a sport or charity event can be a fun team building experience. Nothing will make a group feel more grounded in the same mission than performing well together.   To get to this level, you must practice. The more you practice, the less likely it is that one or more of you will make mistakes during performances. Practice until every group member has the routine/songs memorized and can perform them flawlessly each time. Group meetings are a great way to make sure everyone is on the same page and that all of the group members are happy with they way things are going with your group.  Making sure that every member feels like their opinion matters is important for keeping a group together, and a group meeting is a great platform to make that happen. Talk about upcoming gigs and events to make sure everyone knows their obligations. Use these meetings to divide up the work between group members. This can be anything from buying supplies, to coordinating with a venue, to advertising etc. Make sure that if there are any problems in the group, they are discussed when everyone is there. If there are conflicts in the group, remain calm and treat everyone with respect.

Summary:
Boost morale. Do activities together. Hold regular rehearsals. Hold group meetings.