Is there a way to change the phrase "The requested group does not exist or you do not have permission to view it" to something custom?  Ideally I would like to to say something to the affect of "only paid members can view this forum."

Thanks.

Those are simply strings in the sp theme you are using... you can customize them however you like...

take a look at the group view template file spGroupView.php of the sp theme you are using...

as usual, we strongly advise you to make a child theme or your own theme before making any changes so you dont lose the changes if you update our themes...