Using string_to_table function in WHERE clause

1) Put the IDs into an Apex collection in a PL/SQL process:
    Assume :P5_USER_ID_LIST =  '45,4932,20,19'
declare
    array apex_application_global.vc_arr2;
begin
    array := apex_util.string_to_table (:P5_USER_ID_LIST, ',');
    apex_collection.create_or_truncate_collection ('P5_ID_COLL');
    apex_collection.add_members ('P5_ID_COLL', array);
end;

2) Then change your query to:

SELECT * FROM users u WHERE u.user_id IN 
(SELECT c001 FROM apex_collections
 WHERE collection_name = 'P5_ID_COLL')

As simple as that...