You can add as many users of each role as you'd like. If the users are already in the course then you simply have to edit them in the Classlist Editor and change their permission level.
Note that permission levels in WeBWorK are hierarchical, meaning that any permission level will always inherit the permissions of all lower levels, so for example promoting a professor to admin will always let them do anything a professor can do.
The default permissions can all be found in defaults.config. Any customizations should be made in localOverrides.conf.