Collaborate ~ External vs. Guest Access

What is the difference between External User Access and Guest User Access?

https://docs.microsoft.com/en-us/microsoftteams/guest-access

Create a team

The next step is to create the team that you plan to use for collaborating with guests.

To create a team

1. In Teams, on the Teams tab, click Join or create a team at the bottom of the left pane.
2. Click Create a team.
3. Click Build a team from scratch.
4. Choose Private or Public.
5. Type a name and description for the team, and then click Create.
6. Click Skip.

We'll invite users later. Next, it's important to check the site-level sharing settings for the SharePoint site that is associated with the team.

SharePoint site-level sharing settings

Check the site-level sharing settings to make sure that they allow the type of access that you want for this team. For example, if you set the organization-level settings to Anyone, but you want all guests to authenticate for this team, then make sure the site-level sharing settings are set to New and existing guests.

To set site-level sharing settings

1. In the SharePoint admin center, in the left navigation pane, expand Sites and click Active sites.
2. Select the site for the team that you just created.
3. Click ... and choose Sharing.
4. Ensure that sharing is set to Anyone or New and existing guests.
5. If you made changes, click Save.

Invite users

Guest sharing settings are now configured, so you can start adding internal users and guests to your team.

To invite internal users to a team

1. In the team, click More options (***), and then click Add member.
2. Type the name of the person who you want to invite.
3. Click Add, and then click Close.

To invite guests to a team

1. In the team, click More options (***), and then click Add member.
2. Type the email address of the guest whom you want to invite.
3. Click Edit guest information.
4. Type the guest's full name and click the check mark.
5. Click Add, and then click Close.