Jira SM had a free option for monthly subscriptions up to 3 agents. However, you can grant browse and comment permissions on projects to Jira Software users without incurring any licensing cost. Reach out to Atlassian for a free instance of Jira Service Management associated with your existing instance if you have more users counted towards your Jira Software.
Also, I would advise as an alternative (far less expensive and without sites, users, and agents limit) to start a free trial of the Customer Case for Jira app. It is also a portal for your external clients where they can submit their issues or ideas. It can be public (visible to everybody), private (restricted for specific email addresses or domains), or you can choose the option to make it a support service (private communication with a support assistant).
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