[kh99]

You want Usergroups > Promotions. Add a new promotion with usergroup "Registered User", Posts=50, and strategy "Posts". ("Reputation Level" and "Days Registered" will be ignored). Then choose "Private Member" for "Move User to Usergroup". You can either have the group changed or added as a secondary group by setting "Promotion Type".

Note that users are promoted by scheduled task, so it might not happen the moment they reach 50 posts. The scheduled task promotes users who have been active recently, so inactive users won't be promoted even if they qualify (although if you run the task manually from the Scheduled Task manager, everyone who is eligible will be promoted).