user management: in a group, add button to add existing users/accounts

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Is your feature request related to a problem? Please describe.

1. have some users
2. add a new group via "+" button in left sidebar
3. you are now in the group, empty user list
4. how do I add users to this group?

Or:

1. click on a group with users in left sidebar
2. users are listed
3. how do i add users to this group?

Describe the solution you'd like

• Add a button "Add existing accounts to this group"

Describe alternatives you've considered
Currently, you have to go to "all accounts", edit a user, add him/her to the group. This is not intuitive.

Additional context

Mockups:

The issue names no implementation files, tests, or entry points. Start by locating the user-management group view and its account/group membership handling, then compare them with the supplied mockups; done means admins can add existing accounts from a group and the updated membership is reflected in the group.