You can create a group either by scrolling down on the home screen to "Your Groups". Click on the plus to add a new group.
You can also create a group directly in the transaction list. The transaction list can be found at the symbol with the two arrows, located in the middle of the panel at the bottom.
Once you are in the transaction list, you can see your groups in the panel at the top. To the right of your groups is an icon with a keypad (nine squares in group). Click on that icon to manage your groups. There you can either create a new group, or add purchases to an already existing group.