Hi @ascreel -
When you say "only access through ProSeries" do you mean you can't access them through Intuit link? Is there an error message? Let me know more details. 
Have you tried to re-send an invitation from Intuit Link? If that doesn't work and if the troubleshooting steps don't work, you may need to contact ProSeries for help. An expert can make sure the Auth ID you're using matches the Auth ID your client is signing in with and/or reset the password for your client if needed.