Be clear about the similarities and diffences each person in the primary couple has about what "poly" or "open" means to them; and also how they intend to handle the differences. They should have this discussion before seeking out other partners. The couple should be able to present an absolutely united front to any new partners, they don't have to agree on everything but they do have to agree to disagree and have guidelines in place to deal with their differences.
My qualifications: recently ended a relationship as a secondary to a primary couple / recently changed my relationship with my primary to secondary or "transitional status"
"It is no measure of health to be well adjusted to a profoundly sick society."