There are a few different ways to can go with this, but the most important thing is to create balanced tribes. You want each tribe to be a combination of strong and weak students... The strong students will learn the skill of teaching others and the weak students will learn how to be stronger students.
As I mentioned earlier, you may want to have tribes be just guys and just girls. It keeps the flirting down, it keeps the girls from being dominated by the boys. The myth that girls can't do math is due to one reason: BOYS! Right around the fourth grade, girls start to notice boys... and, for some strange reason, they think boys won't like them if they are smarter than they are... So, they start to dumb-down. Math and science are the first things to go and this often sticks for the rest of their lives. Let's break that cycle now and let the girls know that they can be just as good in math, if not BETTER!
A suggestion for tribe formation:
1) Go by their grades from their last math class. Make yourself a little deck of cards -- each card with a student's name... Put all "A" students together, "B" students together and so on. Decide how many tribes you want with how many members (4-6 members is good). Then, deal the cards out like this:
You may want to have them fill out an info sheet (especially if you teach at the college level) to get this and other info. I always ask them questions about how strong their study skills are, how excited are they about being in college and how strong of a math student they perceive themselves to be.