Someone that is paired at least once in the month will count towards your number of active users. This way, we bill for unique users and not number of pairings or how many seats there are in an organisation. If people are added to the group but leave before ever getting paired in that month, you will not be billed for them. Likewise if someone joins later in the month and get's paired, or they leave the group after having been paired earlier in that month, they will still count towards your total number of active users.
A person only counts as 1 active user, no matter how many times they get paired. Since people can only be linked to one Dewdropz pairing group(the last one they were added to), it means you will only be billed once per unique user.