I am receiving strange behavior with the CRM resolving contacts for
appointments.

If I create an appointment and invite nobody (i.e. I am the organizer but no
atendees) and then track in CRM, it works fine. Appointment goes into the
CRM and I am set as the Owner and Organizer and there are no required
atendees.

If I create another appointment and invite 1 person who is a user (not a
contact) and then track in CRM I receive an error indicating that "No
resources have been selected for this activity" and asks me if i want to
continue. I click yes so it pushes it into the CRM. When I go to view it in
the CRM it shows the required person (the user that I invited) as being
unresolved. It also shows the organizer (me) as unresolved even though it
was able to resolve the owner (me again).

I have double checked that I have no other contacts in the system with the
same e-mail addresses as our two users. Only 1 user each. Why would it not
resolve those users?

We often create internal meetings between users.