You can add existing users to an existing domain license. The pricing for this is $5 per user per year (for 5 or more additional users), or $10 per user per year for 1-4 additional users. Additional users on an existing domain license may be purchased using this link:
Note that page will show a price of $10 per user per year until you have requested 5 or more users, and then the price given will be $5 per user per year.
Adding users to an existing domain license, if you kept the original expiration date, would result in the additional users paying for a full year of use, but only getting a partial year. To handle this issue, we extend the expiration date to prorate the additional value according to the following formula:
D = used days of the existing license
U = existing user count
N = new users being added
(The new license will be issued for U +N total users)
New Expiration = Today + [ (365 - D) * U + 365 * N] / [U + N]
Example: An existing domain license with 5 users is extended by an additional 10 users. The existing license has been active for 48 days.
New Expiration = Today + [(365 - 48)*5 + 365*10] / [5+10] = Today + 349 days