Your units can wear up to 3 items. You have to give the item (with the +1 to the bond you like to boost) to a unit that does not have the bond-buff.
So like you said there are 6 units with the bond-buff for "Greed Demon".
You have to get the +1 item for "Greed Demon" 2 times (which will not happen in every run).
Then you have to put 8 units onto your fiel (6 from "Greed Demon", 2 from other bonds).
Give the 2 units without the "Greed Demon"-bond-buff the +1 item for "Greed Demon". Now they count as "Greed Demon". (Do not put duplicates from the "Greed Demon"-bond-bugg on the field. They already count as "Greed Demon" and will not be affected by the item.)

Hope this helped!
Cheers

Answer to your request in my comments for the "Greed Demon" achievment:
There are items that you can to boost your bond from 6 to 8. They can drop at any battle, but are more common after boss battles. You can also buy them occasionly in town. You should upgrade your town to the maximum for this.
They give you a flat +1 to this bond.

╔═══════════════════تَكْبِيرالله☪اللهتَكْبِير ════════════════════╗
If you are a beautiful strong Mujahideen, someone will put this in your comments.
╚═════════════════لَا إِلٰهَ إِلَّا الله مُحَمَّدٌ رَسُولُ الله══════════════════╝

yo

And to you too, be in good health

With coffee and tea, observing that which I cannot see.