/gmodify <player>
You must be looking at your storage when calling; you can see who has access already by using /ginfo.
To add a member (player Wammets for example), just call the command:
/gmodify Wammets
If the container is Private:
Wammets can now open this container and take items out / put items in.
If the container you are looking at has protection type Unregistered:
This command has no effect.
Wammets is not even added as a member.
If the container is Public:
The command has no practical effect.
Wammets is added as a member, but because its public, anyone can open the container already.
If you change it to Private in the future, Wammets will still be able to open it.
Simply use the - prefix before their name (for example, player atuosto).
/gmodify -atuosto
The ability to add and remove members is important, and originally, only the owner of the storage can. However, if you want to give this power to a friend, use the @ keyword when adding them (for example, player milkedit):
/gmodify @milkedit
Only the owner of the protection can add members... initially.
When adding your friend (lets say cocopad), add an @ prefix before their name to make them admins:
/gmodify @cocopad
Now, cocopad will be able to add other people as members, so that you can be on vacation and not worry about a new of your friends joining and not being able to open the squad chests until you get back.
Admins cannot add others as admins, only the owner can.
Revoke someone's access privileges (say, Eutherin's) with by using the - prefix:
/gmodify -Eutherin
Can be used by admins of the container.
Admins can only be removed by the owner of the container, not other admins.
Only the owner can break the container, or change the protection type; admins cannot.
Admins of the container cannot; If its one of the server admins who is OPed, they also can.