Of course Yuna's the leader. It's her pilgrimage, right? The game focuses on Tidus simply because he's the got the coolest story (next to Auron's, but Auron has to be the mysterious cool guy who decides to help out). Auron can't be the leader because he's not there for the whole journey (I think he joins around Luca, right?). But, he is the most experienced, so he's guiding the group, telling them where to go. Here's the breakdown as I see it:
Yuna: the nucleus of the group, without her there'd be no group at all.
Tidus: The motivator, whose high spirits and optimism spur them on.
Auron: The experienced master, departing words of wisdom, advice and guidance.
Wakka, Lulu, Khimari: The shields, whose primary (and pretty much only) function is to protect Yuna, as well as to provide company and other social needs.
Rikku: The extra hand, providing a little more assistance if need be.