There could be one who is quiet or ill at ease within a group. The group should be mature enough to not tease or make fun of one who seems ill at ease in a group setting or even more quiet or is simply less experienced on a backpacking trip. All should be made welcomed and encouraged to be part of the "family" during the trip in the great outdoors.
Try walking two abreast for awhile. If the trail is not too wide, but is wide enough to walk two abreast, some people are more at ease in very small group. It might be a good idea to mix the walking partners up every so often and let the slow person set the pace, so that everyone get to communicate with everyone in the group. If you can not walk two abreast, try mixing up the hiking order up the best way possible. I realize that the lead person may have to always be in front, but he should also keep an eye out as to how everything is going within the group every so often. Discord can be like a knife in the back, so all should be careful not to employ rude comments about others in the group.
Talking behind peoples back is just as bad in group setting and should be culled out at the very moment it starts, perhaps by simply changing the subject matter. We all have to be mature on this issue as well.
When setting up camp, the various chores (if necessary) can be done in in small groups or partnered up. If there for a few days, let the chores and/or groups be alternated, so as to avoid the "why me all the time" scenarios, while at camp. The leader should also lead by example and perhaps do those things the others find less agreeable. Make sure that any work load of whatever there is does not fall on the shoulders of just one individual either. We have to share this experience together.
If you have a campfire going at night, try to encourage everyone to participate in setting an exciting mood with stories, songs or just talking about the events of the day. Do not pressure anyone into doing more talking because they are simply quieter than others. Some people are better at this than others, and others prefer to listen; but if participation is fostered correctly, with respecting one another as a family would, then I believe that a sense of community would be achieved and as well as a sense of being needed will go a long way.
This is simply my two bits with my thoughts on this question. By the way, Charlie, this is a great question and I am sure others will answer it better than I have.