Here's an example from UNC. The main idea is climate change. The student groups information into three areas, evidence, causes and solutions. They draw out lines to different smaller groups of information that support an area. This can be simple or more complex depending on how much information you need to organize and visualize.
Teach and test
Take turns teaching the material. This can be as simple as explaining a concept or it could be teaching how to work a problem.
Create your own tests. Go though the material and create short tests for the rest of the group. Alternate taking and making tests.
if you have to memorize...
Try to start 2-3 days before the test
Read over your notes, but don't cram on the day of the test
If you are memorizing from written notes, try to write out the notes without looking at them. Then go back and see what you missed.
If you are memorizing from cards, use different colors for different kinds of information on your flashcards. For example, green might be problem solving steps, blue might be terminology, etc.
In a group
Use mnemonic tips, like rhymes, songs, and using words with the first letter of the item you're memorizing. You can even try visualizing a story. Teach each other the mnemonics.