i know when you have a full party you get increased drops etc but seeing as i can never get a full team to run one boss for hours on end can the free avatars people register be used as a sub

Yes, but at a lower bonus.

• 1 Player (ie. just you), 0 NPCs: 0%
• 1 Player, 1 NPC: 15%
• 1 Player, 2 NPCs: 20%
• 1 Player, 3 NPCs: 25%
• 2 Players, 2 NPCs: 30%
• 3 Players, 1 NPC: 35%
• 4 Players, 0 NPCs: 40%
• 3 Players, 0 NPCs: 30%
• 2 Players, 0 NPCs: 20%