Cooperatives are exclusive or "member only" workshops that you will encounter on the overworld. They grant skill level bonuses to their members respective to which type of workshop they are.
For example, Blacksmithing cooperatives grant a bonus of +20 to the blacksmithing skill while standing within the bounds of the workshop. This bonus stacks with any other skill-based bonuses you may have, such as from equipping an Ancient Smith Hammer which are obtained from Angry Miner camps.
Each type of cooperative has a number of locations around the map. In order to enter a cooperative you must become a member.
Note that these structures are NPC owned and do not have any friends or co-owners.
To gain access to a cooperative you must first attain a membership deed for it based on what type it is. These are rewards for the completion of Bulk Order Deeds. Make sure you are standing directly underneath the sign of the cooperative when targeting it with the deed.
You will see a message in the bottom left corner of your screen indicating the expiration date of your membership. You can always check the expiration date again by clicking the workshop's house sign.
You can increase the duration of your membership at any time by simply adding more deeds to the sign. Note that you will need to add a new membership deed to each different cooperative you wish to join, even if it is the same type as one that you are already a member of.
The strongbox is a virtual container similar to a bank box in functionality and is located inside each cooperative for its members. Items you store within it will be accessible and visible only to you.
This container acts as a regular strongbox for co-owners in that it only allows 25 items to be placed inside and has no weight limit, making it ideal for bulk resources.
Contents of strongboxes are deleted upon membership expiration. Be sure to renew your membership prior to expiration by adding a new membership deed if you plan to continue to use that cooperative, as they are stackable.