My students are planning on taking an exam together via Zoom
I've spent some time thinking about the problem.
Disclaimer: I believe exams are a way to push students to learn something from the course, grades being an incentive. The solution below may be very unorthodox, so brace yourselves. It's also based on my experiences at an European university, where an instructor has a lot of power in defining an exam and grading it.
I see two ways the Zoom meeting can work:

They solve exams individually, then meet-up for a review session, correct the exams and send them. Which is a good strategy. They will mostly work on the exams alone and during the review session they will gain additional knowledge.
They meet up just after the exams are given and spend time solving them together. It's a bad strategy. Brain storm sessions are usually ineffective, some of the students won't be pulling their weight and, I guess, in the end they will have to spend more time on the exam to turn in the same quality answers as working alone.

That being said, recognising your own weaknesses and turning to someone for help is also a very important skill in life, so I wouldn't mitigate it. I also like very much Buffy's idea to make them cite the sources.
So this is what I'd do:

I'd ask them to cite the sources. When working individually, the sources should differ. If they work together it will be a lot of trouble to not only change the answers, but also the sources.
I'd allow them, however, to work together and ask them to send a contribution report. I'd expect that it would encourage them to divide the workload more equally.
I'd set a different grading standard for individual vs. collaboration work. E.g. the points you lose on a mistake is weighted by a number of people that work on the exam.
I'd allow them to mark one reviewer (whose help doesn't affect a score) and their contribution. I'd encourage the reviewers to also report on their reviews. If the contribution was helpful, it would improve the score of the reviewer. If possible I'd also make myself available as an reviewer.

I believe these solution would change possible ways to cheat into available strategies, making the exam also an interesting experience of collaborative work.