Collaborating with external partners is an inevitability for many organizations today. However, with all the security issues that’ve occurred in recent years, it’s understandable that some might not be especially welcoming when it comes to sharing data or letting others into their Microsoft Teams tenants.
With guest access, you can provide access to teams, documents in channels, resources, chats, and applications to people outside your organization, while maintaining control over your corporate data.
https://docs.microsoft.com/en-us/microsoftteams/guest-access
Sometimes you'll want to collaborate with people who aren't in your team’s organization. If you want to collaborate with them in Teams, you can add them as a guest.
This article describes how to configure guest access settings - including calls, meetings, and chat - in Teams. Guest access in Teams also requires configuring other settings in Microsoft 365, including settings in Azure AD,
https://docs.microsoft.com/en-us/microsoftteams/set-up-guests