This is for the convenience of the teacher. Record the teams that students choose or are assigned when you do this project.
If at all possible, each team should have at least one person in the class who cheers for that team. Obviously, this won't work for classes smaller than 18. For larger classes limit the number of students for each team to a maximum of three.
For a downloadable online version of the sign-up sheet, go to
http://compellinginput.net/p/free-stuff