You can do that with hashtags, but it is only going to show up within your instance's federation. Gnusocial has ! Groups which do propagate all posts, but mastodon didn't implement that bit.
Matrix.org has a good model for channels (called rooms), groups of channels [1] (called Communities), and distributed identities. I only hope that Matrix.org will become the common background middleware for all federated services, and all these will tend to implement the same features for interoperability.
[1] https://medium.com/@RiotChat/communities-aka-groups-are-here...