It depends a little which code cycle you are on, but the following should be sufficient in most cases...
1) If there is not already a grounding electrode at the garage, install one.
Run a grounding electrode conductor from the inverter to the grounding electrode.
2) Bond all the metal parts of the PV modules and racking, and run the equipment grounding conductor (EGC) to the common ground bar in the inverter.
3) When you install your inverter output circuit back to the main service, you'll run a grounding conductor with it. Size this grounding conductor according to 250.66 or 250.122, whichever is larger. Make sure, at both ends, that the conductor is bonded both at the inverter or breaker location (so that it meets the needs of an EGC), and also to the grounding electrode conductor or grounding electrode (so that it acts as bonding jumper between the two grounding electrode systems). You may need to irreversibly splice on a pigtail to accomplish this.
One additional thing applies if you are on the 2008 code cycle.
690.47(D) Additional Electrodes for Array Grounding.
Grounding electrodes shall be installed in accordance with
250.52 at the location of all ground- and pole-mounted
photovoltaic arrays and as close as practicable to the location
of roof-mounted photovoltaic arrays. The electrodes
shall be connected directly to the array frame(s) or structure.
The dc grounding electrode conductor shall be sized
according to 250.166. Additional electrodes are not permitted
to be used as a substitute for equipment bonding or
equipment grounding conductor requirements.
To follow this, you may need to put an additional ground rod at the location where the wiring comes down from the roof, if it is not at the same location as the inverter.
