Hi,
The guest users permissions are limited in Microsoft Teams, but they have the ability to participate in private chats (by default they cannot add or remove channels or tabs), Teams conversations (for joined Teams), and have access to files and the SharePoint information behind the Team as well (they will also be Guest users of the Office 365 Group).
Some more information:
Microsoft Teams guest access checklist
https://learn.microsoft.com/en-us/microsoftteams/guest-access-checklist
Best regards,
Leon