Hi Robert,
We have done this using the RoleService() with the internal API. Something like:
CommunityServer.Components.RoleService roleService = Telligent.Common.Services.Get<CommunityServer.Components.RoleService>();
//need to get/create csUser object above here
roleService.AddUserToRole(csUser);
This worked really well for us, I'd imagine the recomended approach is to use the webservice though.
Thanks
Adam