To add additional storage to a free Personal Edition account, you will need to upgrade your Personal Edition account to a paid account. The maximum amount of storage a Personal Edition account can have is 50GB.

For a Business Edition account, you can add an unlimited number of users and storage to your account.

Log into http://my.syncplicity.com as an Administrator within your company account, and then click on the Console tab.  A summary of your Business Edition account is provided under Plan Info which includes the following details:

 
Seats - To create additional user accounts, you would simply click on the link Add New Users. If you want to purchase more seats or reduce the number of purchased seats, click on the Add Seats link.

Storage - If you want to purchase more storage or reduce the amount of purchased storage, click on the Add Storage link.