Thank you for your answer. I have tried out several variations during implementation of the Group Self-Selection plugin.
I got it working, sufficient for my purposes.
What I have done is, I created 2 Moodle Activities on course level:
1. a Group Self-Selection (plugin) called: "IV-B: Join a Practice Group".
2. a Forum called: "IV-B: Private Forum".
The screenshots in the Moodle documentation do not correspond with the latest version of Group Self-Selection module (3.11 , 2016090800):
The initial setting-options of minimum / maximum members per group have been reduced.
Later on in the implementation process, these parameters are difficult to adjust!
It was not necessary to make a special Grouping, so I removed this work-step from my To-do list.
Grouping only makes the workload for the admin heavier, as he/she must check every course for new student sub-groups and then manually add these groups to the grouping.
IV-B: Join a Practice Group
Purpose of a personal sub group :
The purpose of a Practice Group is to provide a private sub group (within this course) in which you can exchange information (text, files, pictures) with two other participants.
Thus, you can practice online with other students regarding the exercises that belong to this course. Be aware that certain exercises may become quite personal, so use a password to protect your practice group.
Join or create a Practice Group :
Find a group that needs an extra member, or start your own group. If a group is still open for subscription, but locked by a password, then you need to ask the first member (= creator) for permission. You can use the Messages link in your Profile menu to contact this group-initiator.
You can only join 1 practice group at a time. To join a second group, you have to leave the first one.
Post in Private Forum :
In the Private Forum area (see General Overview: click on Private Forum), you will find all posts and files that belong to the Practice Group of which you have become a member (or a creator). All personal material is also accessible by the teacher and the admin.
Course completion :
When the last person leaves the sub group, this practice group will be dissolved and all data will be destroyed. So, if the three of you have completed the course and want to say farewell to CPER, please first leave your practice group, in order to remove all traces of personal content.
IV-B. Private Forum
If you want to share information with certain students only, or upload exercise-files for practice reason with a sub group, then this is the place to be. But first you must join a Practice Group with two other participants. You may need a password from the group-initiator.
You can also create a Practice Group yourself. Then invite two students to form a training sub group for data exchange between the three of you.