Most of the time it is hard to use two different values in command argument, I think the better way is to use only one variable but separate it with some separator
see below snippet
//use it like
CommandArgument='<%#Eval("AssignGroupId")+","+ Eval("UserId")%>'
and then
string[] commandArgs = e.CommandArgument.ToString().Split(new char[] { ',' });
string AssignGroupId = commandArgs[0];
string UserId = commandArgs[1];