I want to incrementally add to a multi-select list the value of the assignee field upon saving its selection so I can track all people who have participated in the resolution of the issue.
You'll need code to do that, which you can't add in OnDemand.
However, I'd add the Jira Toolkit plugin, and then use a "participants" field from there. It'll accumulate current reporter, current assignee and anyone who comments on the issue, which is going to be pretty close to what you need.
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