How can I assign students to groups manually

The PRO version assigns students to groups during the generation. It can even change the group for student if more gorups are availalbe (unless you lock the students in that group of course)

The regular version only assigns the students before the generation then it cannot change the groups.

In both cases, you might want just to assign the students into groups, without generation, so that you can see the cards, perhaps play around with them to see how they stack together.

In this case, just press this button:

Alternativelly:
you can manually assign any student to allowed group.
Select one or more students, then right click on the column with subject you want to assign the students to and then select your group:

Just remember:
1. this distribution might not be optimal. For example if you have two subjects that all students must pick. The software might divide first subject as boys/girls and second as Older/Younger students. Then you will not be able to combine these two subjects. This is very simple issue, things might get more tricky if you account teachers time-offs etc. The PRO version can change the students during the generation so it may find out its good to divide the students into same groups for both subjects.
2. if you are happy with the groups, you can lock them: