WhatsApp group calls are about to behave more like Zoom calls and other more free-form chats. The company has started rolling out the ability to join group calls in progress, rather than having to be there (and stay) when the call starts. You can drop in and out at will as long as you were invited at the beginning.
There's also a call information screen to show who's participating in and invited to a chat. Also, don't worry if you have to hit "ignore" for a call — the door will still be open if you want to join later.
The addition is a recognition that Zoom and other more flexible video chat services are still popular even as the pandemic (hopefully) winds down. WhatsApp is also acknowledging that some people have always had different video chat habits — this could make the service more appealing if you've always left a video chat running so that friends could pop in when they have a moment.