Since the very beginning, we both agreed on the rule of always using condoms for penetration; it took us years (about four years) of bonding with another couple to start even considering the discussion of not using condoms with them. When I say bonding, I mean not only sexually but at a personal-friendship level.
The four of us discussed the option during sober moments and made the decision, and planned it accordingly, including an STD test for all. Later, perhaps a little more than a year later, we decided to include a mutual friend couple that we play regularly; the process was the same with them, but this time involving six of us. Since then, all six of us have agreed to get tested at least twice a year and share the results.
We have never regretted our decision, quite the opposite, we love our “exclusive group” and the deep intimacy we share with them.