(not enough space in the title to finish sentence)
From the announcement : A party containing mulitple Guild members gain bonuses, depending on the number of players in the same Guild:
2 Guild Members: +50 STR/INT, +100 DEF, +1 Critical, +10 LUK
3 Guild Members: +60 STR/INT, +150 DEF, +1 Critical, +10 LUK
4 Guild Members: +70 STR/INT, +200 DEF, +1 Critical, +10 LUK
5 Guild Members: +80 STR/INT, +250 DEF, +2 Critical, +10 LUK
6+ Guild Members: +90 STR/INT, +300 DEF, +2 Critical, +10 LUK
The stat bonuses are only available to the guild members in the same party, NOT the entire party. There is an unlisted 100% AP and EXP bonus (can't remember, not sure about the EXP bonus) from having at least 2 guild members in the same party, and this bonus applies to everyone in the party.
E.G. Lets say you have a 6 member raid party, only 2 of which are in the same guild. The entire party will get a 100% AP/EXP bonus, but only the two players in the same guild will get the +50 STR/INT, +100 DEF, +1 Critical, +10 LUK bonus.
Would be great if Saygo or someone else could confirm whether this is a bug or working as intended.