Only Team owners can add guest users to Teams groups.

Guests do not have to belong to the UV (they can belong to another organisation), but they must have a professional or educational Office 365 account. In case email does not have a Microsoft account, from the same page of access to the group, it gives us the opportunity to create such an account.

The procedure to add a guest is as follows:

In Teams we will open the Teams group we want to manage and go to the administration options and choose the option 'Add member'.

We will write the e-mail address of the guest to join the Team and we will select it in the window that appears below the e-mail we are introducing. We will click on Add to "e-mail" as a guest with which we will have the user available to add it definitively to our group, but before we are allowed to edit the guest's information, that is to say with what name it will be shown inside Teams. Finally, click on Add and we will have added the invited user to our Test Team. The user we are inviting will receive the invitation via email, which will contain the link to the Team to which we are adding them.
If the invited account does not have a business or educational account Office 365 as is the example Gmail account, Microsoft warns us that this email address does not have an account with them and allows us to create one from the same page.