Assigning open issues to coverers or adding them as watchers
By clicking on the people icon in the Action column a dialog opens that shows all open issues applicable to the rule.

There are two actions you can choose:
Assign to coverers
Add as watchers
Bulk assignment
After choosing which issues you want to reassign or add watchers, simply hit the blue Confirm button.

How to reassign existing issues that are not open
Problem Statement
By default, only the issues that have statusCategory In Progress can be reassigned. This means that issues in the backlog, in to do, or completed, can't be reassigned.
Solution
However, if you need to reassign additional statuses, simply create a rule with a JQL filter and include those status with the following syntax:
status IN ('The first status you want to have', 'the second status you want to have').
The dialog will then take these.