Allow Admins to Add Existing Users to a Group

[kimcoleman]

Is your feature request related to a problem? Please describe.
Site owners have requested a way to add existing WordPress users to a member's Group.

Describe the solution you'd like
A way on the Edit Member or Edit User screen for the admin to "add child" OR a special "admin only" section on the Manage Group page to "add existing user" to the group.

Describe alternatives you've considered
The current best workaround is to use a plugin like User Switching to go through checkout as the target "child" user, using the appropriate Group Code to claim a seat in the group.

The caveat here is that this will send an email about the membership change to the target "child" user. The workaround for this if you do not want that email to go out is to disable the "Checkout - Free" email, either sitewide or while you take these actions momentarily.

[MaryOJob]

A customer requested for this feature.
Moderators Only: [#609178]

[MaryOJob]

A customer requested for this feature.
Moderators Only: #616698