How to check if an user invitation has been accepted by the external user in Sharepoint Online.
You can check 'Access requests and invitations' under site setting.
answered Apr 22 '17 at 20:00
If the external user accepts the invitation, will appear in the user suggestions in places such as 'check permissions', people picker, etc. basically, the user won't be added to the site collection user list till first successful login.
If you are looking for a email notification to you when your invitee accepts it, that feature is not available.
