i don' know the email-adress of Barbare Brun, but i think i can help you anyway
So i write this in English assuming that you understand this much easier than German especially the labels of the buttons.
So choose your course, look for the block 'People' (only visible to admin and teachers i think) and choose groups. Look to the upper right corner, there is the button 'Turn editing on'.
After this you can edit your groups, like selecting and removing students to a group.
Hope I could help you.
PS: Also ich hoffe, mein Englisch war verständlich.
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Da war ich wohl nicht schnell genug