If using Active Directory, then at login (i.e., one of your AD authenticated users accessing your Gallery URL), they will assume the default role you have set up for them. From the Help page...
New Gallery users assume the default user role unless you add them to a group or assign a user role assign them a role other than Default.
If you want to have users assume a role other than the default user role, you can add them to a group and assign that group a role. Or, after they have registered, you can change their user role.
Detailed instructions are available at the Add Gallery Users page. In your case, if you want certain users to be assigned the role of Artisan, and others to be assigned the role of Curator, this can be accomplished by Creating a Group, add Users to that Group, define the appropriate Group Role (Artisan or Curator), and once they login, they will assume that role. I recommend reading the page linked directly above - this has Step by Step instructions for performing this process.
