Additional info:

• The ready button works with all kind of raid passes
• The ready button works in both public and private lobby
• The ready button will only be enabled when there is 2 or more players in lobby so no instant solo
• The ready button disappears when there is 15s left
• When all players are ready the countdown will move to 10s instantly
• Once the lobby hit 20 players the countdown will move to 10s instantly. However players are still given a chance to leave without consuming a raid pass

Credit to u/Ox0c on Reddit