I'm up and running with Teams as guest user at Another company's Teams-site, but I can't set up any new Teams for other Communications.
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
As LeonLaude said, the guest experience has limitations by design. Here is a list of some of the functionality that isn't available to a guest in Microsoft Teams:
*OneDrive for Business
*People search outside of Teams
*Calendar, Scheduled Meetings, or Meeting Details
*Create or revise a team
*Browse for a team
*Upload files to a person-to-person chat
*Currently, Teams supports only State 1 and State 2 types of guest users as defined by Azure B2B
5 people are following this question.