Need advice for table design for multi-user access
I'd try creating a group and user_group table. Then change map_user_id to map_group_id and your group all have access to manage that item.

Eg.

+------------------+------------------+
| primary_group_id | group_name       |
+------------------+------------------+
|             1    | Edit Stereos     |
+------------------+------------------+

+---------------+---------------+---------------+
| user_group_id | user_id       | group_id      |
+---------------+---------------+---------------+
|          1    | 1             | 1             |
|          2    | 4             | 1             |
+---------------+---------------+---------------+

+--------------+-------------+----------------+------------+-----------+
| map_group_id | map_item_id | unique_item_id | item_owner | privilege |
+--------------+-------------+----------------+------------+-----------+
|            1 |       100   |              1 |          1 |      NULL |