Let's say I was traveling with a friend. My friend is flying economy whilst I'm flying first class. Could I visit the lounge, get the food and all that jazz, then exit the lounge, give my boarding pass to my friend and let them check the lounge out to see what the lounge life is all about?
In some cases, your status may allow you to bring in one or more guests. This is very specific to each airline/program and the various levels, though, so you'll have to check the details for your own loyalty program.
This page gives some general rules. They say that access via your travel class (business, first...) usually doesn't allow you to bring in a guest. On the other hand, most programs (but not all) allow you to bring a guest if you get access through your frequent flyer status, with a few conditions and exceptions, though. So, again, check the specifics for your combination of frequent flyer program / status / airline / type of flight (domestic or international) / class of travel.
Note that the number of guests is usually limited. Even as a Flying Blue Platinum member, I can't (or couldn't the last time I tried) bring more than one person in a lounge, for instance.
However, giving your boarding pass to someone else so they can enter without you is most probably a breach of the terms and conditions of the program.
In recent years, I have encountered with all major airlines that they want to see your boarding pass and a valid ID.
As a consequence, you can of course walk out and in as often as you like, but someone else with your boarding pass (and ID!) will have trouble if he doesn't look a lot like you.
It is also a clear violation, and if caught, you will not only lose your access right, but also potentially be disqualified from their frequent flier club, losing any miles already collected.
As others mentioned, depending on airline and status you might be allowed to bring a guest, but they have started to be a lot less generous there too - just because you are Platinum Select member with an international business class flight doesn't mean you can bring your wife (with an economy boarding ticket) for free
Legally no, but often there is just a check of the ticket (and a scan of ticket, so that you may enter only once).
You might be able to pay an entry fee (maybe with a discount because of your eligibility). It is said that it is cheaper than staying in the "normal areas": you have less time to wait near boutiques and restaurants, so possibly less expenses.
Could you? Yes. Should you? No.
Better for you to load up on snacks in your bag, get a drink in a red cup/fill a bottle, and leave.
I flew 100k a year until recently and regularly brought doggie bags out. I also got to take guests, and would grab total strangers for fun. Sometimes it was good, sometimes they thought I was a creep. To each their own.
Edit: I'm just going to leave this here: If you hand your ticket to your friend, they go into the lounge- what ticket do you have? Your friends? If the TSA stop and decide to search you or your friend, what ID are you going to show? Is it reasonable to explain you traded tickets so your buddy can get into a lounge? Sure- TSA or the police might not blink, but they are going to pull them out to look and be sure. Just not worth it for a cheap brownie. However the Sushi in Japan on the other hand...