In an effort to better manage and monitor the security of our Office 365 environment, we have enabled a monthly external access review for all Teams owners who have active guest accounts in their Teams. If someone is listed as the "owner" of a Microsoft Team that has at least one external guest account in its membership, and your Team is currently listed as "active", that user will receive a monthly review request in the form of a Teams chat notification from the ShareGate Apricot bot. The notification should look very similar to this:

When you click on the external sharing review, you will be able to review the external-facing content (such as shared documents) and any guest users. You will then have the ability to disable any external content links (this will only remove the external sharing, it will not remove the documents themselves) and also remove external guest accounts from your Teams if they no longer need access. If the guest accounts and content are still valid, you are free to ignore the ShareGate notification, and you will be prompted again the following month. More detailed instructions on the process to review external access can be found on the ShareGate help article here: https://teams-management.sharegate.com/hc/en-us/articles/360060998411-Take-action-on-external-sharing-and-guest-access

In addition to this monthly review, we also have setup a page on the IT Service Catalog that explains all of our policies for external guest accounts

https://keminind.sharepoint.com/sites/ServiceCatalog/SitePages/external-o365.aspx

The important highlight of our policies is that guest accounts will be audited on a monthly basis for activity. If a guest account has not logged in within 90 days, it will be disabled. If the external partner still needs access after being disabled, a help desk request will need to be submitted asking for the account to be re-enabled. Any guest accounts disabled for 1 year will be deleted completely and would have to be re-created if still needed after that point.