The SAML Single Sign On plugin for JIRA Data Center / Server allows Single Sign On for the JIRA Service Desk Customer Portal (https://<jira-baseurl>/servicedesk/customer/portal). To activate the customer portal redirection, you have to enable the Redirect SD Customer Portal checkbox in the SAML SSO configurations under Redirection:

In general, customers can be created and updated during the SSO login, using SAML Attributes received from the Identity Provider (same process for normal users → see Create or update users through SAML Attributes). In addition to the SAML attributes update, customers will also be added to the group specified in the configuration field JIRA SD Customer Groups:

It's not recommended to add SD Customers to a group giving application access (e.g. jira-servicedesk-users), otherwise licenses will be consumed for these users.