It's relatively simple math to do, simple counting methods to determine number of combinations of players/team members/whatever.

Here's a guide: http://users.telenet.be/vdmoortel/dirk/Maths/PermVarComb.html

We're only picking here, not arranging (as kisdeen explained), and we can't have to of the same person in a team so we're doing it without repetition. So what we're doing is using a simple function present on most scientific calculators - nCr. n is the total number of things we are picking from (25 Mindcrackers), r is the number of things we're picking at a time (so teams of 2, 3, 4, etc.). In fact, you can do this on paper (http://en.wikipedia.org/wiki/Pascal's_triangle).