You can upgrade your plan at any time in order to add more users. Remember that only active users count towards your license threshold.
To upgrade your plan simply follow the Upgrade Subscription link where you can add more users to your plan.
Once you have your additional licenses, adding users is as simple as clicking on Add User for each individual user, or if you want to perform user additions in batch, feel free to reference the Importing users from a spreadsheet or Importing users from Google Apps domain.