Rule 1. Never ever ever do anything like this with friends you have known beforehand as conventional friends. It is great making new friends who share your interests, but the relationship you have with them will always be different.
Rule 2. Never ever do something like this behind the back of your partner. If you feel you have to, then your relationship is not strong enough, and may end, so be aware of the risk you are taking.
Rule 3. from our experience, swinging (or experimenting if you prefer) works when and only when both people in a relationship have a strong enough love between them to understand that fulfilling a naughty desire is just that. If you really love your partner, you have certainty in that love, if you dont have certainty, don't complicate your life by risking your relationship.
Believe it or not, if you both have that certainty, then involving others can actually strengthen your relationship, for then there are no secrets. But, if either of you don't want to do something, then no is no, never ever think it can be changed by persuasion, love just does not do that.
By the way, this is written from experience and from within a loving relationship, founded on certainty. If you aren't in that space, then don't judge those of us that are.