Define the jobs that are in the project team and have students prioritize their interest in each job. Select teams to have a balance of students all high in interest in a different position. See the Circus project for an example of this. Include a disclaimer that you will try very hard to honor wishes, but there are no guarantees. If there is one kid in the class that no one will work with, you're dead if you guaranteed them their wishes.
Many students are uncomfortable working in teams.
Stealing from business, an opening team-building exercise may help bring the team together. One is to give the teams the outline of a crest or shield, or any shape, divided into compartments. One compartment is allocated for each team member, with a central compartment for the team. Each individual draws an image or icon which represents him or herself. Then, the team creates an image that represents the entire team.