Chatting with external users without needing to switch tenants enables you to get the results that you need faster. Let’s take a closer look at how it works.

Depending on the administrative settings of your organization and of the Teams personal account user:

• Start conversations with external Teams personal account users.
• External Teams personal account users can start conversations with Teams at work users.

Click on the below link for more information.

https://techcommunity.microsoft.com/t5/microsoft-teams-blog/microsoft-teams-users-can-now-chat-with-any-teams-user-outside/ba-p/3070832