Players enter a room, they see a bunch of sleeping guards and some dogs. They try to sneak in to get the jump on them, but the dogs have advantage on perception checks and detect them, initiating combat.
Everyone rolls initiative and the fighting commences. The battle makes some noise, and NPCs from side rooms hear this, and decide to jump into the fray.
Do I as the DM just roll initiative for these NPCs and stick them in the initiative order whenever the NPCs detect a threat?