There is not an option for this. Users need to have an account to open issues. They can be automatically added as @Alex Christensen indicates. If they only ever use email for opening/commenting on issues then not login required but if they use the portal then they need to login.
...be more productive while being fun to use at the same time. For some, getting started can be a bit intimidating. This is especially true if Jira Service Desk is your first exposure to Atlassian...
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