That 4+ party member boost stats for trusts will be useful when trusts are allowed in alliances, trusts were made for single use modtly, so I think the bonus should be the other way around, the less peeps in party the stronger the trust.
<<Alter Ego Levels Relative to Players In Party>>
Players Alter Ego Level
One (Solo) Average Item Level of Summoning Player +1
Two Average Item Level of Summoning Player +2
Three Average Item Level of Summoning Player +3
Four Average Item Level of Summoning Player +4
Five Average Item Level of Summoning Player +5