I'm sure that you have already looked at the student help guide for groups, but in case you haven't, it might help. The Student Groups video tutorial also is very handy. You could make an announcement that explains the need for your students to sign-up for a group and put a direct link to the user groups sign-up page for the course in the announcement. This would help students to know "where to go" in your course to sign-up for the group activities.
I usually make any course registration/orientation requirements as assignments to be completed with a grading rubric attached. For instance, I teach chemistry, and my students are required to watch a safety film, complete safety training, sign a safety contract, and take a safety quiz. Each of these items is then listed as part of an assignment in my Orientation assignment group (0% weight). As the students finish the portions of the assignment (i.e. watch the film, complete the quiz, etc.), I update the students' scores. They know that when they complete the assignment, then all the requirements have been met. You can also use the Message Students feature to remind students that they haven't completed all of the requirements. Your students would know that they need to sign up for a group from what you put in the instructions of the assignment, and you can put the link to the groups sign-up page in the instructions as well.

I find that the assignments work well to keep the students on track, since they are listed in the course calendar with the associated due dates. The number of reminders that I have to send out is usually between 2-5 for all of my sections combined, which is about 75-100 students.
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