Contacts from the Account/Lead other than the SR Primary Contact, can be tagged to the SR from the Related Parties tab of the SR detail page.

1. Click on the Add From Existing button to see a list of contacts attached to the Account/Lead.
2. From the list, select the contact you want to add to the SR and the Contact will get displayed on the SR Parties tab.
3. Click on the Remove button against the Contact to disable the association of the Contact to that SR. This is allowed for all contacts except the SR Primary Contact.