I encourage you to read this documentation as it gives a very helpful overview of teams and channels in Mattermost:
Here are the answers to your questions:
- I believe it can only be done via channels.
2., 3. Yes, channels can be created in Team Edition. Administration of channels will be handled by System Admins.
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Here is information on the types of channels we support: https://docs.mattermost.com/help/getting-started/organizing-conversations.html#channel-types.
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We normally recommend starting with one team that includes multiple channels. The documentation that I linked at the top of my message will have more information on the options to organize your channels and teams. Also, if you change your mind in the future and decide to have multiple teams, you can use this CLI command to move channels to a different team: https://docs.mattermost.com/administration/command-line-tools.html#mattermost-channel-move.
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Yes, users can belong to multiple channels at the same time.
Regards,