Mr. 2 touched everyone but Sanji's face, so he could impersonate them, and kill them off when they weren't looking.
They put the X's on their hands just in case, and over the X's they'd tie on a bandana over it. Crocodile managed to figure it out, and had Mr. 2 go impersonate Ussop to trick Vivi, so he could kill her. It almost worked, but they didn't know about the X's under it.
As for the arm-raising scene, Vivi was calling out to them as they said away, asking if they'd establish her as one of their crew. Marines were around, so they couldn't say anything, or she'd get into trouble.
So they show their symbol to get the message across