A user is defined as one personalized email address (email@example.com). Group email aliases (firstname.lastname@example.org) are included free and don't count as users.
Flexible and Annual Plan?
Flexible Plan - You can add and delete users at anytime. You only pay for the service used during that month. You're billed monthly.
Annual Plan - You commit to Apps for a full year and get a discounted rate. You can add more users, but can't delete users during that year without charge.
How do I pay?
You can pay by credit card, or depending on the location of your business, you may also pay by direct debit.
You can also pre-pay by making a manual payment to your account. The credit will be automatically debited at the start of each month, instead of your primary form of payment being automatically charged.