You can purchase additional users in the Premium, and Enterprise plans anytime. 

1. Click your profile picture in the upper-right corner of the Zoho Projects top navigation band.
   
2. Select Subscription Details to view the pricing details.
   
   
3. Click Upgrade Add-ons to purchase additional users.
4. Select the number of users and click Proceed. Click Make Payment to complete the process.
   
5. Click Downgrade Add-Ons to reduce the number of users.
   
6. Select the number of users and click Downgrade. Click Confirm to complete the process.