The conference server tries to add them as soon as possible (i.e. in the IncomingReceived call state). At this stage the participant device is in state ScheduledForJoining.
Once the call reaches the StreamsRunning state, the participant device state will move to Joining is ICE is disable. Otherwise, the server will wait for the ICE reINVITE before changing the participant state.
The server also sends out an update in order to offer the right streams based on the chosen layout.
The participant device state will ultimately move to Present once the client accepted the server offer and the other participant are notified of the addition of the participant device.