Blaze orange and "situational awareness" are most important when hunting with even 1 other hunter...let alone a group. Vest and hat. I usually hunt grouse in thick bush here in Ontario, but only have 1 season under my belt. Seems every grouse I flushed (pushing/no dogs) flew right towards my hunting buddy. I simply don't hunt without it on, or with guys who won't wear it.
Aside from that, the common sense stuff already mentioned. High energy snacks, water, comfortable clothes...even more comfortable boots. (waterproof) Shot size~as mentioned, 7-1/2 or 8. The chokes (fixed) on the guns I use are modified, so if you're lining-up a sitting bird that isn't that far away, aim a little high. (head shot, if possible) Not sure what a full load of #8s through a full choke would leave you on a close/sitting bird. I've never tried a full.