in MonoX there is no way for the users in the group to unsubscribe from this newsletter by default. The premise is that the administrator should control who gets the newsletter based on roles. So when you sent out the newsletter to users in 'Users' role, you've decided to send it to everyone in the system.
The basic use case for this scenario is to create a new role and send the newsletter to all users in that role. Assigning users to that role needs to be custom handled (e.g. adding the users to the role from the administration, or coding logic which will enable users to add/remove themselves from that group - effectively subscribing/unsubscribing from the newsletter).