Hi @PercyGilmore ...
You can create all your groups first via the "People" page in your course. First, on your "People" page, you create a Group Set. This Group Set name could be the same name as your discussion topic title, the same as the name of an assignment in your course, etc. Then, once you have the Group Set created, you create the groups within the Group Set. For example, if you had 20 students in your course, you might want to make five groups of four students each and name each group within the Group Set based on your favorite colors, sports team names, animal names, etc. You could have "Green Team", "Blue Team", "Red Team", etc. You can then put students into each of these groups within the Group Set as needed.
If you later created a completely different Group Set on your "People" page, you could do that, too. You'd again create groups within this new Group Set. Even though you have the same 20 students, you could put them in completely different groups in this new Group Set so that your students have different conversations with other students in your course.
When you create your Discussion topic, you'll want to select the option, "This is a Group Discussion". You will then have a drop-down list to choose the Group Set name that you created earlier. Doing this adds group discussions for that topic. Canvas knows to keep the students in each of your smaller groups together to have their own discussions within the group.
Here are some Guides for you to look through:
I hope all this will be helpful to you. Let Community members know if you have any questions...thanks!