Mumble - Giving Group Privileges
Unless the user is already registered (has a +A icon next to their name, might also be a tick on some newer Mumble skins), you’ll need to register them (or they can register themselves). Right click the user in the tree, and select “Register”. Then:
Right click the root of the channel tree (the “server name”), and select “Edit”.
Select the “Groups” tab.
Pick the appropriate group from the drop-down box.
In the “Members” box (the left of the three), type the user’s name down the bottom. It should auto-complete.
Click “Add”. The user’s name should appear in the group box. If it appears in Italics, it means the user isn’t registered.
Click “OK” to save it.