I generally agree with Seraph0x that he best way to be dom'ed or domme'd is one on one with the right person calling the shots, but I think that there are two other things to consider.
One, this is an attempt to play out a fantasy. I would never suggest that a person who is new to this lifestyle or just starting to explore try any real-life scene involving large numbers of people. That would be overwhelming, many subs aren't clear on their own limits or their own power to say No and it could turn bad really quickly if the dominant group wasn't responsible. And let's face it, with group dynamics and sexual role-play, it's easy for responsibility to go out the window.
But this is virtual, on-line, the sub doesn't actully have to do anything commanded but may simply get off on the interaction with a group of people. So it is a chance to get a taste of the fantasy without the reality or the risk in real-life.
Second, I don't agree that there can only be one dominant person in a scene. I think we're too quick to list people as doms or subs, when in reality I view it as a spectrum. So, I think it's reasonable to think of it as tiers of doms - one in charge of the overall scene, and others taking direction from that person, but free to act on their own within whatever limits are set by the lead dom, and of course the sub.
Likewise I've been in situations with two subs - one given power, by me, to dominate the other. Again, I have the ultimate say on limits, but within the parameters I've set, one sub is being dominated by me and then, based on his or he own desires, dominating the second sub.
Anyway, just my thoughts.
Oh and Indy, expect a PM from me

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Mistress D