Hi,
It is possible also to enrol students into the course and also directly into groups by use of a group enrolment key.
You need to enable the enrollment method called "Self-enrollment" (with password) in the course (as many times as many groups need to be created) and after that you should create as many groups as you need. If the enabled password in the self-enrollment method is exactly the same password as the one of the password groups you created, students will be enrolled to the course and they will be assigned in the group at the same time.
https://docs.moodle.org/28/en/Enrolment_key#Setting_a_group_enrolment_key
Cheers,
RD