Outer space may be a frontier that some future generation will have to cross, and perhaps in 500 years everyone will be relocating there. However, at this time, no one needs to go to space, and many prefer the earth under their feet. There should still be some who explore, though.
I'm not exactly sure what reasons there are to ship 7 billion people into space. As a solution to over population, shipping enough people to make a difference presents a staggering cost (esp. If one considers the cost of setting up self-sufficient colony for such a large population). If your question is, instead, that everyone should VISIT space, I still maintain that the cost of getting 7 billion people anywhere remotely close to "space" (for example, 250 miles above earth) is far greater than any benefits that might be reaped. Basically, anyway you dice it, the cost of getting 7 billion people into space far exceeds any benefits it may offer.
All that being said, it could be said that we are all already "in space" insofar as Earth is in space.