Don't ask me why, but apparently some guy records his adventures during handshake events.
Anyway, he asked Yuukarin and Dawa who the leader was, the second time was for confirmation from Dawa:
#001: Is Kanon the leader?
Yu: No, she isn't.
: So it's still pending?
...
: Who's the leader? Is it Ayaka?
Ay: Yes, it is (lol)
#002: Are you alright with being the leader for S/mileage?
Ay: Yeah (lol)
Just take this with a pinch of salt for now.
Although, in terms of seniority Dawa is the oldest member.