Scratch

"joined the gallery" is such an unclear (to me) concept.

There seem to be very different uses:
   1) bookmarking a gallery to visit it often
   2) having a project in the gallery
   3) having commented in the gallery

Is there a difference between having the project owner add a project to a gallery and having the gallery owner add a project?  Is the project owner a member of the gallery in both cases?  If what you want is "the number of programmers with projects in the gallery" then yes, but if what you want is "the number of people who added things to the gallery" then the answer is no.