Thank you for your answers, but there is more

Very possibly emailing your customers once a month for events not having anything to do with you would be problematic, but it's not 100% clear that it would be: suppose some customers go to the first show invovling you? If they attend a show at the gallery, are they then customers of the gallery? The fact that you and the gallery have a relationship and you would show at least some times at the gallery muddies the water.

However, as to the second part of your question: you can't be bound to an agreement you never signed, and if you've never seen the "artist of the month agrement," it would not bind you UNLESS it is referenced in the coop agreement as another agreement binding signatories of the coop agreement. In that case, the burden would have been on you to request a copy and check it out before signing.

However, if not incorporated by reference into an agreement which you have signed, it does not bind you. Going forward (assuming, for example, you renew w/the coop after the current term or agreement is up), they can require you sign and follow said agreement; but they can't bind you to it retroactively, unless, as I say, it was somehow incorporated into something you did agree to.