Excel online, get user name

Hello,

We are developing an excel online add-on, and I would like to know the user login/email.

Already found, that SSO can give us the user token in the case when the user is well registered in Microsoft.

But we also would like to share the document with SharePoint simply by user email.

How I can get known the user email in that case (from the addon code)?

Thanks and Regards,

Andrew