First, you should communicate about the boundaries and preferences. What do you like? What is not preferred? Are you going to invite a male or female? Who do you want to join you? There are many things you should talk about.
Second, be generous of each other. Monogamy is not appreciated here since you are going to invite another person in your sex life. Forget about it. You should be more generous. Make the third person feel welcomed.I think that won't be a problem for swinger couples.