linq where clause when id is in an array

You can try something like that :

public List<User> GetUsers(int[] ids)
{
    return Users.Values.Where(u => ids.Contains(u.UserID)).ToList();
}

Alternatively to Quentins answer, use this:

public List<User> GetUsers(int[] ids)
{
    return Users.Values.Where(u => ids.Any(x => x == u.UserID)).ToList();
}