How can I use the return value from prev_picked() in my ajax call?

Code JavaScript:
function autosuggest_results (info)
{		
	if (info.length > 1)
	{	
		show();
		prev_picked();
 
		var with_field		= document.getElementById('users');
		var with_auto		= document.getElementById('with_auto');
		var users_list		= document.getElementsByName('with_username[]');
 
		if(users_list.length == 0)
		{
			ajax.post('/assets/ajax/pm_autosuggest.php', 'info=' + info, function(resp)
			{		
				var search 	= eval('(' + resp + ')');
 
				fill_list(search);
			});
		}
		else 
		{
			ajax.post('/assets/ajax/pm_autosuggest.php', 'info=' + info + '&list=' + with_u_list, function(resp)
			{		
				var search 	= eval('(' + resp + ')');
 
				fill_list(search);
			});
		}
	}
}
 
function prev_picked ()
{
 
	var users_list		= document.getElementsByName('with_username[]');
	var with_u_list 	= '';
 
	if(users_list.length > 0)
	{					
		for(var j=0;j<=users_list.length; j++)
		{
			if(users_list[j] != undefined)
			{
				if((users_list.length - j) > 1)
				{
					with_u_list += users_list[j].value + ',';
				}
				else
				{
					with_u_list += users_list[j].value;
				}
			}					
		}
	}
 
	return with_u_list;
}