A group is a conversation between three or more people. Start one from + beside Direct messages, then New group, or from New message in the app. Groups are listed with your direct messages in every team. Everyone in a group can rename it, add people by handle and leave it from Group details.
A document and a podcast recording session each have their own conversation. Choose Chat at the top of a document or a recording session and it opens in a panel beside it, so you can keep writing or recording while you talk. Anyone who can open the document or session can join, and sees what was said before. In the app, Chat is in a document's menu.
These conversations are kept when the session ends. They are listed in Chat under Discussions, named after what they are about. One that has been quiet for two weeks moves behind Show older. A recording guest signs in to use the session's chat; recording still needs no account.