Tap a raid and you can run it alone or take a party. Unlike bosses, a raid party is not drop-in: you either start a party and become its host, or join a party that is still waiting in its lobby. Once the host presses Start Raid, nobody else can join that run.
The lobby lists everyone who has joined, with their combat level, and you can open any member's worn gear and inventory to see what they are bringing. Eat, drink and swap gear in the lobby — the wait between bosses works the same way, 5 seconds while the next boss walks in.
Everyone but the host has a Ready button, and the roster shows who has pressed it. The host cannot start until the whole party is ready — the Start button counts who is still getting set. The host's own answer is pressing Start, and a member who died on the last run is not waited on. Readiness clears when the run starts and again when the party lands back in the lobby. Chat works in the lobby exactly as it does in the fight.
The raid itself is the group boss fight: the server runs every swing, and it plays exactly like a solo run — same tick speed, prayers, specials and combo eating. The bosses come in order, and only the last one pays. Deal at least 10% of the whole raid's hitpoints — counted across every boss, not reset between them — and you roll the raid's reward table yourself, with your own collection log entries, the raid kill count and your slayer task progress. Miss the line and the clear counts for nothing.
Finishing the raid puts the party back in its lobby, so the host can start another run without everyone regrouping. If the whole party is wiped out, the run ends the same way. Dying costs what dying in a solo raid costs.