Flag team
Main flag carrier
Spec for maximum mobility and defence. Offence is secondary.
Gem nearly entirely for resilience and stamina (Yes, this isn't arena, gemming for stamina is allowed as there is a balance between resilience and stamina where there is an optimal level of survivability*)
Here is an example feral build; http://www.wowhead.com/talent#0ZxhGsf0zfeoIIuVk0gcz:jN0V. (If you have any questions about the principles behind it, please ask)
Secondary flag carrier
A standard arena PvP spec would apply here. How they gem should be decided once the team is constructed to see if more survivability is needed or more damage.
Flag healer
Spec for utility and defence. Against a good team you will be primary target after FC
Flag defender
Largely offence here. In fact so much so, that if the healer can cope, a full set of PvE gear would be optimal.
Kill team
A standard 3v3 comp here. Slightly off-topic; this group has one of the more vital and challenging roles to play. They must be able to fight a turtled defence, and survive due to the long distance from the graveyard. They also have to deal with their opposition always respawning, thus giving them less than 30 seconds to deal with the enemy flag carrier against a competent opposition.
Liquid/Midfield Team
These players cannot be constrained to a single gearing and talent method. The only advice that I could give before the composition is chosen is that utility is key. These players must be constantly ready to react to orders and control various points of the battlefield as it changes.
Edit; If you are interested, please sign-up on the guild calender.