get account by access token

Hi there!

here is what I am facing, 

I basically would describe myself a a guy who knows a "bit" about Adobe Sign services, I used  to build apps using old intergration keys,  now I only use oauth with tokens, pretty much no serious issues,

but! 

I am building an app which requires the following,

I need to be able to give clients oauth authorization link (based on my app client id and client secret)

so they can authorize themselves with their own Adobe Sign accounts - so in the end my application can catch access and refresh token for each client account - that bit works fine,

but I have no idea how to get client Adobe Sign email account - login basically, 

image I can get 10 different tokens in database, each for different account, but there is not way to idenfity them, 

so for example if I need to send a document to one of the clients I don't know which token to use

hope I make some sense