User groups are a way to structure the team members in your projects. You can restrict card workflow transitions so that only team members in certain user groups can execute them. For example, if your workflow includes a transition to indicate that story analysis is complete, you may only want team members in the "business analysts" user group to execute the transition. You can define any user groups you want in your project. User groups may be based on roles, organizations, skills, or whatever else you decide.
User groups go very well with Mingle project templates. You can create workflow transitions associated with user groups in a template. The specific users in the projects created from the template will differ, but the workflow transitions will still be applicable for the team members in the user groups.