I'd recommend you to take a look at: https://confluence.atlassian.com/display/Purchasing/New+pricing+model+for+JIRA+Service+Desk+2.0+FAQ and https://confluence.atlassian.com/display/SERVICEDESK/Agent+-+JIRA+Service+Desk that contain the answer to your questions
Agents are your JIRA users who will be able to actually work on issues reported via a service desk portal, as well as see the JSD interface: Queues, Reports, etc. Work on means, update field values, transition through workflow - normal issue interaction.
None agents can be named as Collaborators, who can comment and be engaged to assist with resolution, but cannot interact with the issue as normal. More about the 3 roles:
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