Like the others have said, each user of Visible needs to have their own account which means they will need their own unique email address. The card used on each account can be the same card. Currently there is not an account/sub accounts type setup.
"Billing" is done monthly and automatically to the method of payment on the account. Teh amount varies depending if you are a member of a party and if you have any referral credits.
I suggest you sign up with 1 phone joining a party and using someone's reference to get the first month for $5. Then sign up the second phone, join the same party and use the refence code from the first phone. That way the first phone gets the second month for $5 and the second phone gets the first month for $5.
And don't form your own party or join a small party, join a party with hundreds or thousands of members.