For smaller teams Discord has perpetual audio channels that you can join and leave at will, and group DM (also perpetual) can support up to 10 people in video at a time
One option could be a Google Hangouts Meet room, as once you start a call, that meeting link continues working forever. So start a call, share the link in your Slack channel (perhaps put it as the channel description), and then whenever anyone joins the call they'll see anyone else who's joined.
Another option could be Sneak, though I haven't tried it before. Basically lets you start a call with anyone, anytime—though also seems to snap pictures regularly which might not be so great.
There are a few services that work like the question is describing, an always open/always on video chatroom where people can come and go as they please.
However what is tough to find is one that meets the second criteria, "for work".
I've found that most of these services have been around since long before the COVID-19 quarantine. They've already attracted various different groups, some of which can be NSFW, that you might have to see/sift through on the way to your intended chat.
That said, they're also pretty awesome because they do feel more "real" and organic compared to say Zoom which to me feels more formal, "send an invite about a topic, get in, and get out."
The two that come to mind for me are:
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