Groups could have a settings dialog in which the creator of a group could set, which rights other users have. For instance, shall other users be allowed to add new users to a group?
Currently no such rights management is available.
As an example, the screenshot shows the rights management options which Telegram offers for groups.
It’s hard to do that in a federated / decentralised system that does not guarantee message order and that every message is delivered like email.
Technically it would be possible to have a group with one admin, but it gets hard as soon as you introduce the option to promote other people to also be able to change permissions and settings. (and not all clients get the message of the promotion before the action the new admin took, so some clients would reject the action that the new admin took, because have not received the promotion message yet)
It would be easier to integrate sth like a mailing list api or bot where you can have special groups that run over a mailing list server and you configure the mailing list from inside of the DC interface via a seamless integration, but with pure email it’s close to impossible, unless you want to introduce a chat-multiverse like in a blockchain, but even then I’d doubt that it would work or be a good user experience so I think hosted/mailinglist groups are the way to achieve this.
Also see the other topics about group types on the forum:
maybe there is a way to solve this with cryptography, like: the admin is the one that holds some private key, they when they grant admin right to someone else, they share the private key (or maybe a sub-key? I don’t know much about crypto) so even if you lost the message of the rights promotion, this new admin can verify they are admin because they hold the admin key