MS Teams Invite
Before adding external users to a MS Teams site, your MS Office 365 administrator needs to enable Guest Users for the Teams app in the O365 Admin Center. For details, click here.
Adding an external user to your MS Teams site is done as follows:
If your external user has a MS Office 365 account, you just add his/her MS Office 365 email id
Else, the external user needs to create a personal MS Account/ email id. Then you can add that personal email id as a member of your MS Teams Site.
For more details on adding external users as guests, click here.
Below is an invite email example.