ADDING NEW USERS TO QUICKBOOKS

We have set up QuickBooks, but are having issues adding new users.  The new users get an invite, but it does not work.  The invite has an issue and the new users cannot get into QuickBooks.  We have talked with several different people at QuickBook and had our complaint escalated.  We get an email telling us they know it is a problem with the email invite and the web address, but our invite email still does not work.  Has anyone else had this issue and, if so, has it been resolved?