Since you have a set of unique values, a Set is the appropriate data structure. You can put your values inside HashSet, an implementation of the Set interface.
My lead said this was a more efficient way to do this and it made sense to me, but it just seemed a bit off to put null/empty as values in the hashmap and store elements in it as keys.
The advice of the lead is flawed. Map is not the right abstraction for this, Set is. A Map is appropriate for key-value pairs. But you don’t have values, only keys.
Example usage:
Set<String> users = new HashSet<>(Arrays.asList("Alice", "Bob"));
System.out.println(users.contains("Alice"));
// -> prints true
System.out.println(users.contains("Jack"));
// -> prints false
Using a Map would be awkward, because what should be the type of the values? That question makes no sense in your use case,
as you have just keys, not key-value pairs.
With a Set, you don’t need to ask that, the usage is perfectly natural.
This is O(1) right?
Yes, searching in a HashMap or a HashSet is O(1) amortized worst case, while searching in a List or an array is O(n) worst case.
Some comments point out that a HashSet is implemented in terms of HashMap.
That’s fine, at that level of abstraction.
At the level of abstraction of the task at hand —
to store a collection of unique usernames,
using a set is a natural choice, more natural than a map.