This could be for a few reasons: 

When the user was created, did the administrator set the user’s status as "Active"? If not, open that user's profile, check the Active option and then click Save

When the user was invited, did they receive and accept the invitation within three (3) days? If not, contact to get their account confirmed for them.

If you have a limited number of subscriptions, was the user attached to one of them? If not, under Organization, click the Subscriptions tab and select that user for an open subscription.