Create a new group
Then on top-right, you can use the Create new group button to generate a new Project group.

You will be asked for the name of your group:

You'll also have to give users access rights. You can add several users at once. Click "Add users" on the right when all users you want are selected.

Then you can edit them or remove them with their individual lines. Default access is "Read & Write" so be careful on this.

Once done just click the button "Create new group" on the bottom-right of the screen.
Edit a group
To Edit a Project Group, go back to the Project groups list and click the line of the Project group you want to edit.
You'll go to the same screen as the creation screen, and you'll be able to edit the Project group name, its users, or their rights.
Just click "Update group" on the bottom-right once you're done editing.

Delete a group
You can delete a Project group, including everything inside it, by clicking on the Delete Group red button on the top-right of the screen when you're on the edit screen.
Please note that Project group deletion cannot be undone!