80 people sounds like a charter situation. And a charter may even be available from a smaller airport with cheap (free?) parking and quick security - but it would likely need to be big enough to have a handling agent. Also, imagine there is an irrop on a scheduled flight, and parts of your group get rerouted through different airports, maybe arriving on different days. A quagmire. And with such a large group your chances of some of the luggage not getting their in time or one piece are high.
In any case, you can not expect to get a great deal from a group booking. They are unlikely to give you the advertised price for which they usually sell ~8 tickets for a group of ~80.
I would ask Air North if they want to do a quote for a charter. SEA is close to several airports they fly to regularly (YYJ/YVR/YWL), and I know they do charters to Mexico.
1.800.661.0407 ext 5
From what I hear, Air North is easy to deal with. They may or may not be allowed to fly between the US and Mexico - they would know. Legally, it may be easier to do a YVR-SEA-DGO flight, if you have any passengers boarding in YVR (or YYJ, YWL, etc.). In any case, Air North can't say more than 'no thank you' - and since they are Canadian, they'll surely add a 'sorry' for free. :-)
Please let us know how it all plays out.