There is no usage documentation!
Topic on Extension talk:PushToWatch
You should see a form at the bottom of all pages, preceded by the list of all current watcher. You can put someone account name (login), there, press enter to submit. The page will refresh itself, and changes take effect immediatly.
The "pusher" and the "pushed to" will receive email notifying them of the "forced invitation". You'll find a screenshot on the page https://www.mediawiki.org/wiki/Extension:PushToWatch
Regards !