In earlier versions of AX we had User Relations to connect a user to an employee or a contact for a customer or vendor.
In AX 7 this is actually simplified. It took me a while to find it, but when I did it was pretty obvious.
You can only connect a user to a contact. The type of contact is then related to some type of entity. Preferably an employee, because there are no customer portals any more.
This is what you do:
Go to the User form (System administration > Users > Users) and put the contact in the “Name” field.
If you wanted to change the relation you go to “maintain versions” in the top menu to update the record. This is pretty neat as it is date controlled and a user can change the party relation at a certain time, this also maintains your history.