The OnGrid Tool does not officially support Net Energy Metering Aggregation at this time.
However, multiple meters with the same non-tiered rate schedule, daily & weekly usage pattern (with a TOU rate schedule) and seasonal usage pattern (with a seasonal rate schedule) can be modeled with one proposal. Simply sum up the usage and demand for each meter. Then add the additional customer charges for each meter in the "Electric Rates & Usage" section -> "Charges Not Yet Factored In" sub-section -> "Additional Monthly Utility Charge (not reduced by solar)--Starting Rate Schedule" input. Repeat for the Ending Rate Schedule input in the next row.
For other cases, you can do a proposal for each meter, but please size conservatively to hedge against the utility over-allocating production towards an individual meter.
To go over any of this in detail, please join a conference call.