ArcGIS Online: Cannot accept invitation for <username> and <Invitation ID>. User email must match invitation email

Error Message

Attempting to create a new member using enterprise logins in a SAML-configured ArcGIS Online account fails and returns the following error message:

Error:
Error in accepting invitation for user '<username>', Invitation Id '<Invitation ID>' Cannot accept invitation. User email must match invitation email.

Cause

The 'Automatically' option under 'Edit SAML login' is selected in the organization's Security configuration instead of 'Upon invitation from an Administrator'. When the 'Invite members to join using their organization-specific logins' method is chosen, the 'Upon invitation from an Administrator' option must be selected.

Solution or Workaround

To resolve the error, select Automatically in the organization's Security configuration.

Note: 
Only an organization's administrator can perform the following workflow.

1. In ArcGIS Online, select Organization > Settings.
2. In the Settings page, click Security.
3. In the Security section, select Edit SAML login.
4. Under the Your users will be able to join: setting, select Automatically.

Note:
The Automatically option allows members to join the organization by signing in with SAML or OpenID Connect login. When signing in with enterprise login credentials through the organization's ArcGIS Online custom URL, an ArcGIS Online account is automatically created in the background using the identity provider settings.
When the setting is set to 'Upon invitation from an administrator', an administrator must add members to the organization before accounts can be created for new members.