When you are in the Service Contacts of a case, one of the options listed under the Actions available is "Add From Public List".
Add From Public List is the only method that should be used to add opposing counsel. You can search for an attorney here using their name. The email address associated with that attorney is what the State Bar has in their system.
You cannot add non-attorneys this way. If a non-attorney is not already listed in the service contacts, you will need to e-serve them through traditional methods (like adding them to your Firm Service Contacts list or by adding them to the case's Service Contacts list as a new contact, if applicable in your state).