Previously it was only possible to join a group call in WhatsApp when the call was first made and the phone was ringing, which meant if you missed the initial notification and the group call had started, you were effectively locked out of joining.
Joinable calls changes all that. Now, if someone in the group misses the call when the phone rings, they can still join whenever they like. You can also drop-off and re-join the call so long as it’s still ongoing.
In addition to joinable calls, WhatsApp has got a new call info screen so you can see who is already on the call, and who has been invited but not yet joined. There’s also a new “Ignore” option in the calls tab if you want to join a call later.
Joinable calls is rolling out now globally. Note that only eight participants can be active on a WhatsApp call at one time.