Restrict username autocompletion to "within own groups"

[wiswedel]

For data protection reasons, I'd like to disallow username autocompletion when sharing.
However, there are situations of trust, where I might want to allow autocompletion anyway:

1. The user and I are in the same group
2. The user has given their permission to be listed (opt-in, [Feature Request]: Change "Allow username autocompletion..." to a "user" setting rather than an "admin" setting #12094)

So the request here is an extra option in the admin's sharing settings to restrict autocompletion to users within the same groups:

[wiswedel]

implemented by #19569