There are a few ways to get there, but it is not as simple as just signing up for an aeroplan account.
The easiest way is to buy or upgrade to executive class or latitude plus AC fare for your flight. Each passenger would have to have a qualifying ticket.
The second easiest way is to either buy a membership to the Maple Leaf Club from Air Canada. It is about $350 per year and it allows you to bring guests with you for a fee (around $20) and gives you access to United's Red Carpet lounges and I think Lufthansa lounges as well. You also are entitled to use business class check-in, which can be a real advantage when you have luggage to check-in and you are travelling during a busy time. This is the link to the application form on the AC website.
http://www.aircanada.com/en/travelinfo/airport/maplelounges/documents/application-form.pdf
Also, you can either get the Amex Platinum Aeroplan Plus or CIBC Aeroplan Gold Visa (with the lounge access added on). The annual fee for these credit cards is around $400 per year and gives you privileges similar to Maple Leaf Club but you do not get access to the United or Lufthansa lounges.
I have also heard of some people getting coupons sent along by AC with their tickets for complimentary lounge access, but I think this is pretty rare.
The most difficult way to get access is to join Aeroplan and travel on enough flight segment or accumulate enough mileage within a year to be eligible for one of the upper tier categories: Elite or Super Elite.