[development] user group permissions

[Chris Johnson]

I'm writing a module which requires that the data displayed be specified both 
by user name (no problem, works out of the box) and by a group that the user 
is in.  I currently have 286 such groups, and the number will only grow 
larger.  This is not a role, but rather a permission to see certain rows of a 
database table.

I can't help but think that this has been done before in Drupal.  Any 
suggestions or pointers as to where best to find existing code that does this, 
or otherwise how to implement it?

..chrisxj