Granting permission to a database user to alter database role

I want a database user to be able to alter login, database user and database role from my application. so, i assigned that user to sccurityadmin server role, db_accessadmin and db_securityadmin database roles....By now, the user can add or remove login and database user. However, the user cannot add or remove any database role membership. What am I missing here? What should I do so that the user can create, and alter database roles in the database?

Membership to db_securityadmin should be enough. Can you post the commands that you attempt and the resulting error message. Also, what version of SQL Server are you using?

Thanks
Laurentiu