Yes, that is possible. To do this, the system parameter ‘invitation_only’ => true must be set in the config and a number x of invitations must be manually assigned to interested users

bin/console user config set ‘nickname’ system invites_remaining ‘x’

@Montag yes

Contacts -> upper box below the "Find People" search box at the bottom after the "Random Profiile" link.

However you might need to have a look at https://github.com/friendica/friendica/issues/14388