Rocket.Chat brings channels, direct messages, threaded discussions and file sharing into a team workspace. It can run on your own infrastructure and supports integrations and customisation. Identity management, video integrations, automation and deployment options vary by edition and plan.
The free self-managed Starter plan supports up to 50 users. Community provides more basic messaging and has a different limit of up to 100 concurrent users. The current paid offerings are Commercial, Government and Defense; isolated deployments and professional support belong to paid plans.
Sources: Plans

