There are no collaborative features for Obsidian Sync at the moment.
As for iCloud, you should be able to share the vault folder using iCloud’s built in options. See:
You may need to tap an option labelled something like “Share folder with iCloud Drive” before an “Add people” option appears.
However, for the record, this isn’t officially supported. Do some testing with your collaborators before you run wild with this. I’m not sure how well it would handle editing the same file at the same time, or simultaneous preference changes, for instance…