Groups within Teams Free for shared interests, collaboration, and communication
Hello !
On the free version of Microsoft Teams, you cannot join a community directly from the Teams interface.
You must ask the person who created the community to send you the invitation link. Once you have it, click on it : you will be taken to Teams to join the community.
Make sure you're logged into your free Teams account, and not a business/school account.
You can learn more about joining a Teams community here : https://support.microsoft.com/en-us/office/share-and-join-a-community-in-microsoft-teams-free-426e5f36-d673-4f83-a486-8d1e2d4bc22f
Please let me know if you have any other question !