If your account has Administrator privileges within your organization, you will see an "Account Members" item under the Admin menu. Here will be a list of the current members in the organization. Click the Add New Member button.
Then fill out the resulting form with the member's name and email address. The new user will be sent an email notification to complete the process.
Note that the initial password expires after 5 days. Should the member fail to logon to their account and change their password, send them this link: https://app.glassfrog.com/people/password/new and they will be given another opportunity.