Yes, you can change the number of users at the time of your renewal. You can add additional users any time during your ongoing billing cycle. However, you can only reduce the number of users during your next billing cycle.
No, you need not to buy different subscriptions. With the same subscription plan, you can setup an intranet for multiple companies. There is no limit on the number of departmental sites within any company. However, our plan restricts the total number of named users across all companies.