I actually had this exact situation just about 6 weeks ago. We have a user community and sometimes our internal people need to manually create the user records for the community. The users also need a special permission set for all the functions of the community to be available to them. I was having to manually add the permission sets in those instances, and since I am not always available that is not a ideal solution.

The solution was to have a process builder fire off when the user records were created, and that process builder fired off a flow that added the permission set.

It works wonderfully.