I'd be happy for it to be limited to a single tab and you can only have up to 5 friends able to access that tab. Likewise, you can access up to at most 5 friends' stash (which could result in extra tabs shown on the stash... one for each friend).
This way, for those of us passing items between friends, we don't have to wait until both parties are online at the same time in order to swap items.
The way this could function would be that you put the item you want to give in your stash. There could be a 'assign' option to only allow a certain friend or friends to pick up an item so you know exactly who is going to get the item. Items without someone assigned can either be open to being taken by anyone or only for viewing (can be toggled - set to viewing only by default).
Just my thoughts.