Just a quick question:
if you give a user who does not have the application rights for service desk the permission to add comments to a Service Desk project does this users need a service desk license?
If the user has Jira Software license, the user can have comment permissions without consuming Jira Service Desk license. Jira SD supports Collaboration with Jira Software users.
You can have details here:
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.