Do Lemmy, Mastodon and the other big fediverse projects already support invite-based registration using codes?
you could just put the invite code into the signup application questionnaire, a server admin can make it say "Invite code here:"
the admins could keep a spreadsheet of the invite codes like in Google Sheets, keep track of who was given the invite code and who received it
https://github.com/LemmyNet/lemmy/issues/1777
but admins can do it in the signup application questionnaire and just keep track with a spreadsheet