WhatsApp today announced that it will allow users to join a group call, even after it’s started. This feature will reduce the burden of answering a group call as soon as it starts.
Even if someone in the group misses a call when the phone rings, they can still join later. Users can also drop-off and re-join so long as the call is still ongoing. WhatsApp has also added a call info screen where you can see who is already on the call, and who has been invited but not yet joined. Finally, you can hit ‘ignore’ to join later from the calls tab in WhatsApp.
Source: WhatsApp