Hi !
If have about 40 users in my User manager, an "admin" with at least "WebCfg - All pages" Privileges, and normal "PortalUsers" group users with "User - Services - Captive portal login" Privileges.
When I declare a new group calls 'password', with this Privilege "WebCfg - System: User Password Manager page" and I make an user member of this group, the I'm capable of login in to the GUI and the only thing that's possible is: having the user changing its password ….
So, it works for me.
I can change the password for this user, when I login - its actually the only thing I can do with this user, and logout.
Are you using the last version, 2.1.4-RELEASE (xx) ?