Discussion groups are a major feature of most social networks. Mastodon does not (yet) natively support this functionality, but communicates with other services of the Fediverse that act as such :

Summary [TL;DR]. From Mastodon, you can create and follow groups on Gup.pe, Chirp.social and groups.qoto.org. Lemmy and Mobilizon offer partial integration.

When will Mastodon support groups ?

Natively, Mastodon does not support groups.

A dedicated bug is open since 2016. @Gargron, the lead developer, said until recently that he does not want to integrate it.

Officially, this feature is supposed to be covered by the use of #hashtag. Recently (since version 4.0.2), it is indeed possible to subscribe to hashtags.

However, things could change: the 2021 business report mentions the development of groups as a goal for 2022.

You can also find an active development branch, with screenshots:

We can hope that groups will come in the next versions.

And elsewhere on the Fediverse?

Mastodon belongs to a larger network, the Fediverse, with whom it speaks a common language (protocol): ActivityPub.

Groups are indirectly supported by Mastodon, through external but compatible services.
Here is a non-exhaustive list:

Gup.pe : ad hoc groups

Gup.pe is a public ad hoc group service: groups are created on the fly when you subscribe or send them a message:

• From mastodon, subscribe (by searching in the search bar) to @anything_you_want@a.gup.pe: the group is instantly created !
• Then, if you mention @anything_you_want@a.gup.pe in a Toot (from mastodon), it will be automatically boosted (=retooted) to all members who follow it too.

The disadvantage of this system: there is no admin nor moderation. This service is likely to be quickly targeted by spammers.

Another design issue: this service boosts all replies to the group message, not just new topics. This can quickly fill your feed. This point is discussed here

Note also that Gup.pe is now a co-op which you can join and support financially to influence its development.

Chirp.social & Qoto Groups: managed groups

Chirp.social is a managed group service: unlike Gup.pe, groups are moderated. A dedicated account is needed to create the group and manage it (block users).

Once the group is created, you can do everything from Mastodon:

• Subscribe to it
• Start a discussion. Note that you must first follow a group for a new discussion to be forwarded to subscribers.
• React (like or answer) to a discussion

Qoto is both a fork (=an alternative version) of Mastodon, and a (large) instance of it, dedicated to the scientific community. This version integrates many additional features, including the support of groups, via a dedicated server: groups.qoto.org.

As with Chirp.social, you can create and administer a group on https://groups.qoto.org/. Everything else (starting and replying to discussions) can be done from your Mastodon account.

Lemmy : the Federated clone of Reddit

Recently arrived on the Fediverse, Lemmy is an open source and federated clone of Reddit, the famous discussion site.

On Lemmy, discussions are organized around thematic communities that everyone can create and moderate.

From Mastodon, you change :

• Subscribe to a community: for instance @nom_communaute@lemmy.ml (Lemmy's main instance)
• React (like, comment) to a new topic

However, it is not yet possible to start a new discussion from your Mastodon account. For the moment, you need to create a dedicated account on one of Lemmy's instances.

The reason is semantic: a topic has a title, a body and possibly a link, while Mastodon messages are Notes (messages without title).

I opened a specific bug to propose solutions to this problem. A similar request is also discussed for Friendica (another service of the Fediverse). See also this general discussion about the semantics of messages on Lemmy.

Mobilizon: private discussions & public communication

Mobilizon is a service developed by Framasoft as an alternative to Facebook groups / pages. It allows associations / collectives to organize and communicate without going through GAFAMs.

Mobilizon groups offer several features:

• Private discussions: they are not federated, and therefore require a dedicated account to participate.
• Public events and announcements: you can subscribe to them from your Mastodon account.

Conclusion

Mastodon does not support natively groups, but you can already use third party services of the Fediverse to discuss specific topics with your communities.