1) You can't assign an issue to multiple users. But you could add a custom field which contains multiple users.Then your users can search in JQL if their user is inside this field. (currentUser() in MyMultipleAssigneesField). https://confluence.atlassian.com/display/JIRA/Adding+a+Custom+Field
2) You can do this with Bulk Edit: https://confluence.atlassian.com/display/JIRA/Modifying+Multiple+%28%27Bulk%27%29+Issues
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