What is it?
Guest Cohort is an enrolment plugin for Moodle that mixes the features from guest enrolment with those of cohort sync. More exactly, it allows you to provide quest access to users that belong to a particular cohort. These users will be able to view these the resources of the course, but will not be able to take part in its activities (they can not complete assignments, they can not take quizes, they will not show in participants lists, they will not get emails from News forums, etc).
Where to use it?
You can use it to give guest access to a course to a group of users, without leaving your course open to all guests and without requiring you to giveout password or them to enter passwords.
On example is when you have cohorts of students organized by year of study: 1st Year, 2nd Year, etc. You allow each year to participate in their courses, most likely using Cohort sync enrolment instances. So 1st year students participate in courses for 1st year, 2nd year students participate in courses for 2nd year and so on. This is what you can do with Moodle core. If you install this plugin, you can give students guest access to courses from previous year. For example, in courses for 1st year you also add Guest Cohort enrolment instances for 2nd Year cohorts. This will allow 2nd year students to view resources from previous year, either to study for postponed exams or to refresh their knowledge. They will do this in guest mode, a "silent" mode that will not bother teachers and students from 1st Year.