Tap most bosses in the combat picker and you are asked whether to fight solo or join a group. A group holds up to 8 players against one shared boss; you join the fullest group with room, or open a new one. Raid bosses stay inside their raids, and four bosses are solo only: Ember Tyrant, Ashen Crucible, Venomcoil Matriarch and Blighted Gauntlet.
The server runs the whole fight — every swing, your XP, the food and potions you use and the loot roll. The fight plays the same as a solo one: same tick speed, prayers, special attacks, combo eating and gear swaps, and dying costs exactly what dying to that boss alone costs. The boss's entry requirements still apply to each player, quest, Slayer level and kill count alike.
After a kill the boss comes back 5 seconds later, shown as a countdown. That wait is preparation time, not dead time: eat, drink potions, toggle prayers and swap gear as normal, and the group goes into the next pull ready.