You can go to 'User roles' in the project explorer. Double click on a user role and add roles to the "May grant roles".
You can only create users having a subset of roles of the 'may grant roles' set of the current user.
We do this so you can not create users with more rights than the current user (unless you explicitly define that with the 'may grant all roles'), to prevent security leaks.
Hi Andrew,
You can go to 'User roles' in the project explorer. Double click on a user role and add roles to the "May grant roles".
Good luck!