George allows Candy to become part of the dream of "livin' offa the fatta the lan'" in many ways because Candy is much like them. He is a person that perhaps much of society finds little use for and has disregarded in many ways; he has outlived his usefulness in the eyes of some. Candy, like George and Lennie, dreams of place where he will not be told what to do, where he can be himself, and work on his own terms.
Candy's offer is one of more than money:
"S'pose I went in with you guys. Tha's three hundred an' fifty bucks I'd put in. I ain't much good, but I could cook and tend the chickens and hoe the garden some. How'd that be?"
But Candy's money turns the dream from being some off in the distant future to something being more accessible. As George says:
"In one month. Right smack in one month. Know what I'm gon'ta do? I'm gon'ta write to them old people that owns he place that we'll take it."
What you are talking about is what happens in Chapter 3. In this chapter, I think that George allows Candy to become part of the dream because he thinks that Candy can help make the dream a reality.
The reason that he thinks that is that Candy actually has money where George and Lennie do not. George thinks that they will need $600 to buy the farm (times change, don't they??) and Candy has $350 that he will pitch in. This makes it seem much more likely that they will actually be able to buy the ranch.
On what page of mice and men does candy offer 350 dollars to be apart of lennie and georges dream?