As mentioned its +5/-10. The sp gain from a mob is reduced for every member of the party, but is then increased for each party member in the +5/-10 range. This is calculated separately for every party member.

To give an example, take a 3 man party with ranks 30, 35 and 40. In this case the 35 and 40 characters will get a bonus from both other players in the party, but the rank 30 character will only get the bonus from the rank 35 character.

I know it works like this as I have seem my sp go down when another character gains a rank and gets more than 5 ranks above me and the sp go back up when I gain a rank and get back to within 5 ranks. All in the same guildleve.

However, the sp is also modified by the rank of the mobs you are fighting. You gain sp for fighting mobs of a higher rank then you up to 10 ranks at which point it caps. To take our example from earlier, if the party was fighting rank 50 mobs then all party members would gain the same sp (which would them be modified by party numbers and rank gaps), but if the mobs were 49 then the rank 40 character would get less sp.

To take a second example, in a party of 35, 35 and 36 fighting rank 45 mobs the rank 36 party member would get less sp than the other two.

A perfect party would have all members within 5 ranks of each other fighting mobs 10 ranks or more greater then the highest rank party member. At this point everyone would get exactly the same sp. Again we have shown this doing guildleves, both everyone getting the same sp and then a party member getting less after they ranked up and getting 9 ranks lower than the mobs.