Group admin autocompletion

Is your feature request related to a problem? Please describe.
We are using Nextcloud as a customer portal where each customer organization has their own group and own group folder. Under admin /settings/admin/sharing we checked Restrict account name autocompletion and system address book access to users within the same groups to prevent customers from discovering users from other groups.

Customer service agents from our company are group managers for every customer group but are not members of said groups (to not have the same restrictions as customer group members inside the group folders). It should be possible for the group administrators to share a new file with a customer group or invite the group to a new talk conversation, collective etc. - but it isn't - because you need user name autocompletion to share with groups and you just have autocompletion when you are a member of the group - not a group administrator.

Describe the solution you'd like
Similar to the settings/admin/sharing option
Allow autocompletion when entering the full name or email address (ignoring missing phonebook match and being in the same group)

there could be an additional option:
Allow autocompletion for group admins (ignoring missing phonebook match and being member in the same group)

Describe alternatives you've considered
Making it the default without the possibility to disable the feature would also be an option, because I think it would be intuitive and helpful, if group admins are considered members of the group in general.

Start at the settings/admin/sharing option described in the issue and trace the account-name autocompletion path for group members versus group administrators. Define how the proposed setting interacts with the existing group restrictions, then verify that group admins can find groups for file sharing and Talk or Collective invitations without broadening access for other users.