user guide doesnt mention it but HA FF group membership can be configured at Advanced Services > Redundancy (in 6.3.1.4 at least)
btw, if you are moving from a master / master backup config then HA FF wont work, see the release 6.3.1.x release notes:
Bug ID: 80206
Symptom: The high availability:fast failover feature introduced in ArubaOS 6.3 does not support a deployment model where a VRRP-based redundant master pair (a master controller and standby-master controller) is also configured as high availability active-standby pair.
Scenario: This topology is not supported because the high availability: fast failover feature does not allow the APs to form standby tunnels to the standby master.
Workaround: None
for me it did work for some APs (tested with 5, first time only 1 worked, second time 3) but the ones that didnt work got lost in limbo until they were power cycled. after two tests i felt it was enough and went back to VRRP.