Unfortunately, Confluence doesn't provide the ability to set space watches for another person - see https://jira.atlassian.com/browse/CONF-22313. The CLI can't work around that limitation. It is different for page watches. Here is the documentation for the addWatch action found on https://bobswift.atlassian.net/wiki/display/CSOAP/Reference. Add watch to page or space for a user. Any user can be specified for page watches. Only the current user is valid for space watchers.
You can use this User Macro I wrote to add space watches for multiple users (unfortunately, you have to do it one at a time, but it's not impossible):
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