Guest access in Microsoft Teams allows teams in your organization to collaborate with people outside your organization by granting them access to teams and channels.
Anyone with a business or consumer email account, such as Outlook, Gmail, or others, can participate as a guest in Teams with full access to team chats, meetings, and files.
In this short video I walk through an end-user being added to another Teams tenant as a guest and walk through the registration process on a mobile device (iOS) , signing in and tenant switching.
Additional details and information on Microsoft Teams Guest Access can be found here: https://docs.microsoft.com/en-us/microsoftteams/guest-access