On Lemmy I can join other servers without having to recreate an account.

Not sure what you mean by “join”. You can participate on other Peertube servers, just as you can participate on other Lemmy servers. If you sign up on another server, you will need another account. But currently Lemmy does not support this. PieFed does.

As for Mastodon, The threadiverse (Lemmy + Piefed, etc) is not intended to follow individuals so it does not have that capability. However if a microblog user replies to a threadiverse user, you will be able to see and interact with that as well.

Hope that helps. Happy to hop on a call or something with you to share what I’ve learned for your presentation.