Open-core, or fully open-source with paid offerings (e.g. Zulip -- see tabbott's comment below [0]).<p>Let the DIYers host their own instances, but offer support and/or on-premise deployments for a fee. It seems to be what Mattermost, Zulip, and others are doing with some success.<p>Mattermost also includes use of their mobile notification relay servers in their paid offerings, which is compelling, because otherwise you have to build <i>and distribute</i> your own mobile apps.<p>[0] edited to include reference to important comment
Free for personal and small team use to gain brand and product awareness, then charge for support, additional features, higher usage limits, etc.<p>Basically the same as Slack, Trello, etc.<p>Be careful with per-user billing though, as this puts a lot of big enterprises off.