You can join it if your group doesn't participate in gnw at that time - talk to GNW NPC.

Most servers have 2 rounds on Wednesday and 1 round on Saturday so they will be only able to participate in 2nd Saturday round.

If you have more than 2 active groups in GNW and you are in active group, you won't be able to participate in this activity.

I don't know how this works with x-GNW. Probably same principles apply.