Yes, it's not that hard. Pezdog is right, they're looking at your communication skills more than anything. Be polite, be friendly, and smile.
They'll tell you a little about the company (when I did mine they went over rates of pay and things like that as well). We were paired up and had to ask our partners questions, then introduce them to the rest of the group. We also did a group task, where we were given a scenario (EG there are 3 trolley registers open, and 3 on express. one of the express people is new and slow, the trolley lines all of a sudden get very long. one person who is experienced on the registers is doing an urgent but simple task somewhere else in the store, then a customer spills tomato sauce all over the floor and it needs to be cleaned up right away so no one slips. what do you do to solve these problems? -> my group said to get the new person to clean the spill quickly, then go and do the simple but urgent task, get the person who was doing that on a trolley lane, call for help from another department if you need to, or something like that.) We also did a short test - really easy. Very basic maths (normal cash handling stuff "$20 - $7 =" type thing).
Group assessments aren't hard. Just be friendly and polite. And at a Woolworths interview it's best to wear formal clothes. Wear something very neat, simple, dark colours. If you want to make yourself stand out wear a brighter colourful tie or necklace or something - just keep that part very simple. Managers at Woolworths used to complain about people who wore jeans or sneakers to an interview. They tend not to like it.