Office 365 now provides guest access that can be used to grant content access to team members outside of your organization. However, the process of setting up a guest user works differently from that of setting up a normal, licensed user from within your organization.
Although within the Microsoft 365 Admin Center you will now see Guest Users option under the User main menu item, you are not able to add guest accounts here. You can only manage or remove them.
To create a guest user account, you must first expand the Admin Centers menu item and then choose Azure Active Directory. Within the Azure Active Directory Admin Center choose to open the Users section. You can create a guest account by entering their email address and type a message to send them an invitation. The recipient(s) will need to click the ‘Get Started’ within the email they have now received and accept use terms. When the guest user completes the registration process, they are logged into Office 365.
You can now add that guest user account to data sources, some apps and notifications.
Updated: Click on the following link to get more details on controlling guest access accounts: https://docs.microsoft.com/en-us/microsoftteams/teams-dependencies#control-guest-access-in-azure-active-directory