System.Collections.Generic.List<string> lstUser = new System.Collections.Generic.List<string>();

int numberOfOnlineUsers = Membership.GetNumberOfUsersOnline();

foreach(MembershipUser u in Membership.GetAllUsers())
      
 // Membership.GetAllUsers will returm MembershipCollctions which holds individual Membership User objects 

 {
    if(u.IsOnline) 
   { 
     lbOnlineUsers.Items.Add(u.UserName); 
   } 
 }