Graystripe is not dead yet, when he dies he will join StarClan.
oakheart came and brought her to starclan.
While we don't know for sure whether Hollyleaf joins Starclan or not, there are three main theories on this topic. The first theory says that she does not join Starclan because of her actions just before she died. The second theory says that she will still join Starclan in spite of this. Another theory says that she is not dead at all, but is trapped beneath the tunnels.
The Darkest Path.
Graystripe died of old age in The Broken Code series, having lived a long life as both a warrior and later as ThunderClan's deputy. He passed away peacefully, surrounded by his friends and family in StarClan.
It is not confirmed when, but it will happen in Omen of the Stars
In the warriors series, the cats that join Starclan are those who die and have followed the warrior code, and served their clan faithfully.
Mosskit did join Starclan as you will learn (if you haven't already) in Warriors Field Guide: Secrets of the clans. He died while Bluestar was bringing Mosskit, Stonekit, and Mistykit to their father: Oakheart of riverclan.
If the cats are good even if they don't believe in StarClan they go to StarClan, but the cats like the cats from BloodClan don't. I'm not sure about the cats that have never heard of StarClan, but are good. Even if some cats believe in StarClan, but they do some really bad things, then they go to the "Place of No Stars".
Yes, when Bone, who was killed by many apprentices, from BloodClan, killed him.
Firestar will join StarClan when he loses his last life. yes firestar join's starclan after the battle in fading echoes he had one life then he died of Greencough so brambleclaw becomes bramblestar also Jayfeather visits StarClan and the Place of No Stars, when he goes to the Moonpool and sees the faint outline of Firestar. Yellowfang explains he will be able to see and hear when he fully dies, and they see the faint outlines of other cats, "dead and long forgotten". that's what she said to jayfether so finally he could become a warrior but he needs a apprentice.
no he is firestars best friend for gods sake so he doesn't die main characters best friends never die but graystripe does get stolen by twolegs before they leave the forest so it takes a while for him to get back so firestar thinks hes dead but leafpool does not see him in starclan
She died saving her Clan and the others from a pack of dogs by throwing herself into the gorge