It's not really different than using Git to work on the same branch. If you and the teammate commit to the same branch, then you'll need to resolve the divergence somehow, usually a merge or rebase, or choose to forcibly overwrite the other's changes.
To be clear, I am describing my actual usage of jj. It’s worth noting that there’s currently not a `jj pull` command, so divergences typically involve manually setting a branch pointer for me.