Here's the way these things generally work:

1) Sign up for the site

2) Complete sponsor offers (usually free trials of things you'd better  
remember to cancel), sometimes at least one offer is difficult or  
expensive, but in any case, you generally have to complete a certain  
number of them in specific groups of offers. i.e. two from group 1,  
two from group 2, and 1 from group 3, as an example. Sometimes they're  
things you might be interested in, but as I say, seems at least one is  
difficult, expensive, or crap.

3) Refer a certain number of other people to the site. This is  
sometimes, though not always, a requirement. There are actually sites  
that trade or sell or buy such signups. And, each of your signups must  
also complete the same offers as you to become active and count  
towards your "gift".

So, everything's paid by the advertisers who get your trial signups. I  
actually know at least one or two people who have cashed in on this  
stuff, but it ain't easy, and you'd better keep track of those free  
trial expiration dates. It's way too much trouble for me.