It's 5 ranks above and 10 ranks below you. As far as we know, that hasn't changed since the end of November.

Source: Comprehensive Version Update Details (11/25/2010)

The rank range that allows for bonus skill and experience point within a party has been expanded from ±5 to +5/-10.
Ex. In a party of three players, ranks 10, 15, and 20, the rank 10 player receives bonuses equivalent to that of a two-person party, as only one of the other players is within the rank range. From the perspectives of the rank 15 and 20 players, however, both of the other two party members fall within the rank range. They will therefore both receive bonuses equivalent to a three-person party.