If I understand your question correctly, you are asking about the basics of UML class diagrams?
Would like to know if its possible to
hold multiple references of an
aggregate root (groups) inside another
aggregate root (calendar) member ?
Classes can be associated with any other classes, in any way they want. That's the whole purpose of designing a system.
Also how a list of references are
represented in UML ? it is a simple
A list of references can either be aggregation (has a) or composition (owns a).
But assignations must hold multiple references to groups of students. (Must work two sides)
This isn't really clear, you are talking about a bidirectional relation between groups and students? In any case, perhaps this article on many to many relations can provide you with some insight.
If there was no such thing as “student”, would there be meaning to “student group” ? Probably not.
If there was no such thing as “student group”, would there be meaning to “student” ? Probably.
It is possible (fellowing DDD rules)
to make Groups holding references of
assignations or it need to pass by the
root for accessing groups from
I never modelled a system with DDD in mind, but by reading the previously mentioned article. I would say, try to prevent it when not necessary, if there are no performance issues, access the groups through assignations. That is, if that's the correctly chosen aggregate root.
Apply the "student, student group" example to all your elements to choose the correct aggregate roots.