The answer to all of the above is Yes you can.
Can I add more users to my organisation?
To add more users to your subscription you need to go into
Organisation > Organisation members > Manage Subscription > you can then add however many users you need.
Can I add new users at any time? and how will it be charged?
Yes you can add users anytime you don't have to wait for the end of the month or at the renewal anniversary.
It will charge the additional costs at the next scheduled billing date. So you will be charged the partial amount for those users (so the remaining days of the current subscription for them)
Can I change the payment card?
If you want to change the payment card you will need to select the update payment details button on your subscription page first. Once you have added the new card details then you can add your new users (if you want to do so at the same time)