Old one, but there is one fast trick: send a gmail where is link to user category, tags etc. and let users — like admins and mods — open those links. Then googlebot doesn’t follow robots.txt anymore and will index those.
Rest is, as usual, matter of Google will it follow deeper or forget.
This topic, and quite many similar ones, gives an image here where robots.txt would a strict rule. No it is not. It is just a guideline, nothing else.
And spam/steeling fear is valid, but trashy bots don’t care what is in robots.txt. If something is reachable they will go there.