If they want to pay their share, yes. You cannot pay into the same group with two different cards on the same account. Each person paying in will need to create their own Pay By Group account.
Accounts can be created with an email address, via Facebook Connect, or as a guest checkout. There is rarely a reason for someone else to log in to your Pay By Group account.
NOTE: You can usually claim multiple shares in a Pay By Group. If I want to claim a share for myself and a share for my girlfriend, I can do so when I join the Pay By Group. Of course, that means I'm paying for both!