When an external user registers by invitation, there are a few steps that should be followed (explained here). One of those steps, the final one, is signing into the portal for the first time. However, for this to work, the link from the confirmation mail should be followed. Many users try to sign in without using the link in the mail. Not using this link means that the registration process is technically not finished yet. The user ends up not being added to the Active Directory and therefore not being able to sign in.
The mail with the link looks like this.
