In order to have the upgraded participants feature, iOS users need to install the 2.20.50.25 beta update from TestFlight while Android users can install 2.20.133 beta version from the Play Store. If any of the call participants aren"t on the same version, they won"t be added to the group call.
Now, when you click on the call button in a group, WhatsApp will ask you which contacts you wish to call if there are more than four participants in the group. Otherwise, if there are four members or less in the group, all participants will be added to the call.