More analyis (groups)

Rewritten-docs.md

@@ -482,7 +482,29 @@ duties related with lecturing of labs are already associtated with supervisors,
 so this role seems not so useful. In addition, no one requested more than three
 level privilege scheme.
 
-@todo: groups, they can be public and private and why is that, what it solves, explain and discuss threshold and other group features
+School labs are lessons for some students lead by one (or sometimes two or
+three) supervisors. Students have the same homeworks and supervisors are
+evaluating them. This organization has to be carried into the new system.
+Counterpart to real labs are virtual groups. This concept was already discussed
+in previous chapter including need for hierarchical structure of groups. Right
+for attending labs has only a person, who is student of the university and is
+recorded in university information system. To allow restriction of group members
+in ReCodEx, there two type of groups -- _public_ and _private_. Public groups
+are open for every registered users, but to become a member of private group one
+of its supervisors have to add that user. This could be done automatically at
+beginning of the term with data from information system, but unfortunately there
+is no such API yet. However, creating this API is now considered by university
+leadership. Another just as good solution for restricting membership of a group
+is to allow anyone join the group with supplementary confirmation of
+supervisors. It has no additional benefits, so approach with public and private
+groups is implemented.
+
+Supervisors using CodEx in their labs usually set minimum amout of points
+required to get a credit. These points can be get by solving assigned exercises.
+To visualy show users if they already have enough points, ReCodEx groups
+supports setting this limit. There are two equal ways how to set a limit --
+absolute value or relative value to maximum. The latter way seems nicer, so it
+is implemented. The relative value is set in percents and is called threashold.
 
 @todo: explain instances why they are useful what they solve and also discuss licenses concept