This is not a theme issue. You need to keep in mind that Mingle is just a theme, i.e. it changes the appearance of your web pages. Other than the slideshow, Mingle doesn’t really have any functionality; all functionality is provided by WordPress or by any plugins you install.
The functionality you are asking for may be provided by the BuddyPress plugin, or by an add-on to the BuddyPress plugin. The best place to ask questions about BuddyPress is in its dedicated support forum. → https://buddypress.org/support/
It’s no use asking me about BuddyPress, I don’t use it so I know next to nothing about it. ¯\_(ツ)_/¯