The ask: Is there a way to send a custom notification (email) to all of the owners of all of our 365 groups that have guest users?
I have an Excel spreadsheet that lists all 365 Groups with Guest users, the Owner(s) of each group, the # of guest users in the group and the apps used for that group.
I believe what our security and governance department is looking for is a way to email the owner and say, for example, "You are an Owner of the 365 group IT - Support and this group has 3 guest users. Please reply and provide justification for access granted to the guest(s) and/or remove the guest access if it is no longer needed."
Answering my own question here. We're going to use our Sharegate management tool for this. There's an External sharing review feature under Policies that allows you to send either an email or Teams chatbot message to the Owners of groups with guests and it includes the name of the group, as well as the number and names of the guests.