Granting model access to a User Group using SQL API

I am trying to write a T-SQL procedure that calls the udpSecurityPrivilegesSave procedure to grant access for a particular model to a user or a user group. The first parameter of this procedure - SystemUser_ID - is the one that I'm not sure about the meaning of. Can this parameter represent a User ID or a User Group ID, or is it for User IDs only? If the latter, then is there any way to grant access for a model to a user group via the SQL API?

One more related question: where can I find the definitions of Securable_Ids / Securable_Names?

Thanks!

Chris.