Thanks! It appears that I am sending the invite correctly and they receive the email. The invitations also show in Manage > Members > Invitiations, but when the user clicks the link to join from the email, they get:
"Access to this place or content is restricted. If you think this is a mistake, please contact your administrator or the person who directed you here."
However, I am unable to make them a Member or even change their role since they do not appear in Member List and are still in the "Open Invitations" list. I'll contact Jive support.
If they are receiving the invitations but are seeing an error message once logged into the Jive Works Community, I would agree that submitting a case in your customer group as a next step would be best. When submitting the case, please also include the username or email of one of the accounts seeing the behavior.