There are two situations wherein a new user will receive an email without a link.
Multiple Organizations
A few users need to access multiple organizations' InsightStore accounts. If an email is sent to an address for which there is already a user account, permissions for the new organization are added to that user account. The user accesses it by switching accounts. In that situation, the invitation received looks like this:
There is no link, as no registration is required.
No Signed Contract
If the organization does not have a signed contract noted in the database, only the user whose account is designated with User has contract signing privileges will receive an invitation with a link.