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Office 365 Audit Log Search
Using Office 365 Security & Compliance > Audit log search, do you know if it’s possible to report on when a user was added to a specific group? i.e. when a user was added to a SharePoint Online site...
Yes, it's possible, As VasilMichev said you need to narrow down your search to just 'AddedToGroup' in the 'Site Administration Activities'.
SharePoint Group Member Addition Activity Search
In case you are looking for a direct report which offers scheduling and more filtering capabilities to check 'when a user was added to a SharePoint Online site members group' then you can try AdminDroid Reports. Check out the online demo http://demo.admindroid.com/#/audit/sharepoint/reports/22014/1/20.
It should be audited via the AddedToGroup event in the Site administration activities section
Spoiler
Added user or group to SharePoint group
AddedToGroup
User added a member or guest to a SharePoint group. This might have been an intentional action or the result of another activity, such as a sharing event.
AddedToGroup
User added a member or guest to a SharePoint group. This might have been an intentional action or the result of another activity, such as a sharing event.