Given that you have the necessary security permissions, when you select 'Send Invites' from System > Security group the 'Invite Users to Workspace' list window opens.

From this window you are able to see the details of which users/expense users are already signed up to Workspace and the status of the invitation, and you can also invite users who are not yet signed up.

The window used to Invite Expense Users to Workspace is functionally identical, with the exception that it is filtered for Expense users.

Status - displays:

Where email addresses have not been entered for one or more users, you are cautioned to enter an email address for each user you want to invite.

Provided that an email address has been entered for each selected user, on clicking 'OK' an 'Invite Message' dialog is opened where you should enter a suitable message; click OK to send the message to each selected user.