When you share a document or site in SharePoint Online you will find that you and the person you are sharing with will receive an email with a document link.
Have you ever wanted to receive an alert when the person you are sharing with has accepted your invitation?
Well I can’t give you email alerts but I can give you the information that you are looking for.
Start by visiting the Security & Compliance app
Then within the reports section select View report:
Click on the link Office 365 Audit log.
And select Accepted sharing invitation:
Ok this is for administrators only. But at least it may help you with a step inn the right direction.