You've got two grounding issues to contend with- fault clearing and bonding to the building ground.
Faults are cleared by current flowing through that fat green wire you pull in with your feed. At the service, the ground buss is bonded to the neutral buss, to return 'utility' fault current to the utility's transformer over their neutral.
What about transformers? Well, the ground /neutral bond you make at your first overcurrent device after the transformer performs the same function for any 'lost' transformer secondary current.
Yet, the all of your services need to also be tied into the building's ground system. This system- the Ufer, ground rod, whatever - is best thought of as being there for lightning, though that's a bit overly simplified. What matters is that you don't want one end of the building to be at a different ground potential than the other end.
We keep this from happening by tying all our ground rods, Ufers, etc., together. If you do not have serious building steel - not even bar joists holding up the roof or a concrete slab - then you're stuck with running a ground wire back all the way to the existing ground rods.
