The resonance items themselves only drop while you are playing with people.
Resonance can be used on equipment items to make that equipment behave as though that person was with you when the equipment dropped.
So you’re playing with your friend Bill and you find a resonance. Later Bill logs off but you keep playing and the perfect item for Bill’s build drops.
The next day, you’re playing with Bill and you apply the resonance to the item then you gift the item to Bill.
This allows you to share items that you find with people that you frequently play with.