Pass Multiple User via URL to assign on tasks via api

I am trying to create a formula url button so that I can create a task but have majority of the fields auto filled.  One of the fields I want to programmatically set is the assignees.  I have it working for a singular email address (which is recognizes as a user).  But how do I handle multiple users if I want to assign to more than one?  My assigned to field is a user list so I just cant figure out how to pass the multiple logins

Here is what I have thus far:

URLRoot() & "db/" & [_DBID_TASKS] & "?a=API_GenAddRecordForm&_fid_12=" & URLEncode ([Record ID#])& 
"&_fid_6=" & URLEncode("Fancy Design Proposal") &
"&_fid_28=" & "Opportunity" &
"&_fid_23=" & "Opportunity" &
"&_fid_31=" & URLEncode("user1@elitestudioe.com,user2@elitestudioe.com") &
"&_fid_8=" & URLEncode(Today()) &
"&z=" & Rurl()