How do I assign groups manually in a two-stage league?

Yes. In a two-stage league the Creator or a Manager can put teams into groups by hand before generating the schedule, or preview an automatic split by skill or at random and adjust it.

Rules: groups must be the same size and every team must be in exactly one group.

After the schedule is generated, groups can be changed only by resetting the schedule, and that works only while no match has started.

A two-stage setup has a group stage that feeds a playoff bracket, so the split shapes the whole tournament.