CodePen has a collab mode (if you sign up for a pro account). You could load-in p5 from the CDN, set up your project using the ‘hosted’ approach. You can test this out in free (non-collab) mode; you write your p5 code in the JS pane of the CodePen editor.
There are free alternatives to CodePen (other front-end development playgrounds) although I haven’t researched/experimented with those.
EDIT: https://liveweave.com looks like a decent free alternative. You can test the Team Up feature, using the invite link across multiple incognito/private browser windows to simulate a group session. The nice thing is you don’t have to create an account to host or join.
EDIT: For some reason, Liveweave didn’t work properly with the latest p5; it wouldn’t run after the first change I made to my JS/p5 code. I used v1.0.0 as a workaround for now: <script src="https://cdn.jsdelivr.net/npm/p5@1.0.0/lib/p5.js"></script>