linq select from database where ID in an ArrayList

If it's actually in an ArrayList, you should create a List<T> or array first. Then you can use Contains:

// Use the appropriate type, of course.
var ids = arraylist.Cast<string>().ToList();
var tmp = users.Select(a => ids.Contains(a.UserID));

While using Contains on the plain ArrayList may well compile, I would expect it to fail at execution time, assuming users is an IQueryable<>.

List<long> list =new List<long>(); 
var selected = from n in users where list.Contains(n.ID) select n ;

OR

var selected = users.Where(a=> list.Contains(a.ID)).ToList();