Good to voice your fears but desire to explore something new & different (for you). It's how we grow - right ?
In traveling this path you'll end up dealing with several elements of human nature including emotional, physical and sexual. Each of them will require sacrifice on everyone's part - in the hopes of a net positive gain for everyone.
Contrary to what some believe, the sexual part you may find to be the easiest to conquer. That's been our experience anyway.
The emotional will require developing extremely good communication skills. Never a bad thing anyway - right ? Lay EVERYTHING out on the table ! Even what might seem insignificant at first. Talk as much, and for as long, as possible to try to feel secure that everyone clearly understands where everyone elses' fears and insecurities lie. Try to work as a team to build caring, awareness & security for everyone.
The physical is a toughie and can often reflect back to the emotional piece. Accept the fact that nobody can be in two places at once. Some sacrifices will have to be made here. Whatever final form (triad, V, quad, quint etc) you end up finding yourself in, you'll likely end up in a situation where everyone can't be doing things with everyone else ALL the time. It's just not possible. Be ready to deal with the almost inevitable fact of someone feeling slighted or left out. Talk right off - in the very beginning - about the reality of that and start formulating plans on how you will handle it lovingly TOGETHER ! Communicate, communicate, communicate !
Did we mention "communicate" ? <grin>
Best wishes to you and we hope your experience offers valuable life lessons and ends you up in a happy and fulfilled place.