Email can be a tricky thing. Too often messages get stuck in junk mail or deleted accidentally. For awhile we've given users the ability to have an Inkling invite resent to themselves on the login page if they didn't get the registration email.
And now we've provided the ability for marketplace administrators to send their users a new invitation if they need it. In the "manage users" area of Inkling, select the users you want to send new invites to and use the drop down menu to select "Send User(s) a New Invite":