Q:

Add collaborator to forked private repo

I have free GitHub account and forked a private repo. 
Now I'd like to add collaborators to my fork. 
But there's no "Collaborators" tab under Settings page. 
Is this functionality not available for free plans? 

A:

This is probably due to the fact that you forked a private repo.  So, only the owner of the original repo that you forked may share the project with other users - you cannot.  For the same reason, you (probably?)  can't make a fork of a private repo public - only the owner of the original private repo can make it public.