https://www.bleepingcomputer.com/news/microsoft/microsoft-teams-free-version-is-now-available/

Microsoft Teams, the communication and collaboration platform is getting a free version to compete against Slack. With Teams, which appears to be a more compelling alternative to Slack. Microsoft Teams free version is limited to up to 300 users and it's a very useful solution for small to medium sized businesses and project teams. Microsoft says that the users can upgrade to premium version whenever they want.

"Teams in Microsoft 365 includes everything in the free version plus additional storage, enterprise security, and compliance, and it can be used for your whole organization, regardless of size," says Ron Markezich, Corporate Vice President for Microsoft.