[development] user group permissions

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