Microsoft Teams allows you to communicate with other people from different organisations that also use Teams.
If you know the e-mail address of the external contact, you can see if they use Microsoft Teams, by opening a chat and entering their e-mail address. If Teams cannot find that person within your company, it will ask your permission to search externally.

Click on the action and Teams will determine if this person uses Teams.
If found, their name will be returned with the word External in brackets. Click this to initiate a chat.

Now you will be able to chat and call them using Teams

This form of communication is called federated. It’s called that because your organisation and the external contact’s have created a connection of trust between them in order for communication to occur. It is important to understand the limitations of this trust based communication.
Limitations
As mentioned there are some limitations in features when communicating with a federated contact versus a coworker.
Communication Type | Working Features | Missing Features |
---|---|---|
Chat | Short Message Format Long Message Format Emoji’s Stickers Giphy’s Inline Pasting of Images Inline Pasting of URLs with Preview Reactions Replies Share to Outlook Delete or Edit Message Save Message Mark as Unread Translate Immersive Reader Invite other people (internal or external and make Group Chat) | Share Files (workaround possible) Delivery options (Important or Urgent) Schedule a meeting from chat Paste a video from Microsoft Stream Send Praise Use Approvals Integrate with Viva Include an App |
Calling | Make a Direct Call Hold Transfer Mute / Unmute Share Screen Record Forward Escalate to Video | Invite other people to the call |
Meetings | Video or Audio Meetings Share Screen Meeting Chat Share Files PowerPoint viewer Whiteboard Gallery Views Background Blur & Replacement Mute / Unmute Attendee controls Presenter controls Reactions Raise Hand Transcription Translation Together Mode Meeting Notes | No Missing Features |
Files | No working features unless external person is a guest in a team | Unable to share files using OneDrive for Business Integration Use OneDrive directly to generate a share link (if allowed by your IT administrator) |
Note that federated contacts are different to external contacts using Guest Access.