This was actually just released a couple days ago, but the docs are not up to date yet. The endpoint is /oauth/connect/userinfo
It will return an OpenId Connect 1.0 compliant response that looks like
{
"sub": "1",
"email": "email@domain.com",
"given_name": "Bradley",
"family_name": "Booth",
"middle_name": null,
"global_user_id": 54,
"infusionsoft_id": "email@domain.com",
"preferred_name": null
}
sub
is the local id of the user in the app. We will get the docs pushed up soon.