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As the role suggests, watchers are not displayed in the list and they can't cast votes.
Creating a Game
To create a game click on the appropriate button:
A form for the new game will appear.
All fields are self-explanatory.
In the "JQL" field you need to specify a JQL filter from your JIRA instance.
All issues from that filter will be imported. You can remove some of them if necessary:
Once you are satisfied with the estimation backlog, click "Start".