How is Arena Opposition picked

[WickedSilk]

It doesn't.

That's why I said I don't believe a word of it
(I find it unreliable at best)

It is also nonsense because any half decent programmer wouldn't make something so dependant on the team. Either the calculations would be unnecessarily slow and/or complicated or it would be easy to cheat. For example, what happens if you get a new party member? What if someone leaves and comes in again? What if all leave and join a new room? In a random order?

[lareth]

Hmm... the simplest thing that I can think of would be when the match starts, sum levels of players (l1 + l2 + l3) and divide by 3. Then search +/- 5, and if cannot find in 20 secs, than take the first free one (or make range 10, and so on...). It shouldn't be to slow to calculate. Any changes in the team wouldn't matter, because you count and match every time matching starts.