Ok, I'm very late to this thread and most of what should be said already has been, so I'll be brief.
I'm not one to dismiss someone because of age as I think people are as old as their intellect and experience makes them, and clearly you are beyond your years in many respects. But I do want to reinforce an idea that's been touched on in this thread that I think is vital in considering your situation.
Each of you are individuals. Each of you are different. Each of you have different motives for engaging in the game and possibly different hopes for the outcome. Each has made certain choices about what they're disclosing to the group about their interests and hopes and it's important to remember that people rarely disclose their every thought and feeling. Each individual also has abstract emotions bubbling up from the subconscious that may or may not be understood or articulated by the particular person (yourself included) to the group. Each member of your group also experiences the friendship in different ways and in such a close knit circle social dynamics are ever shifting and changing according to circumstances, perceived connections, emotions, influences from outside the group and so on. And finally, there's the levels of trust involved and how good or bad each of you are with regards to disclosure and confidence. For example, how would the group or the individuals be effected should word of this game get out?
Because four individuals coming together causes all of these factors, I want you to understand that no level of disclosure, communication, or understanding can completely protect against all outcomes. The hope is that you, your friends, and your friendship will continue without tension, conflict, or hurt, but with so many variables it's vital that you understand that it's a gamble. You simply cannot know for sure and you should reflect on whether or not you want to put your relationships on the line like that. Your friends should also ask themselves if it's worth the threat it poses to your friendship. Because better or worse, sex changes any social relationship (just like any other shared event).
Never forget that everyone experiences life in their own unique way and people invariably surprise each other. I have no intention of telling you what to do but I do want you to carefully consider what's at stake.
Cyran0