Upto 16GB Free Space you will be getting for lifetime in Dropbox.
To get upto 16GB Free Space in Dropbox you need to do simple task that is you will have to refear or invite your friends or any other user to get signup up with your Refearal link.
For each signup by your referal link you will be getting 500 MB Space for both who signup with your link and for you also.
So upto 16GB you can get Free Space by allowing a user to sign up with you referal link.
But keep in mind with only signup you will not get the Space, once signedup you should tell them to Install Dropbox on computer and signin after complteing this steps only you will be credited with you reward free space.
One benefit is you both will get extra space so the user also will be ready to signup with your link because he is getting 500MB additional free space.
So this referal process you can do untill you get 16GB of Free Space.
Than you can enjoy 16GB of Free Space in Dropbox for lifetime.
If you want to get Addional free Space in your Dropbox than signup with this link : http://db.tt/L3sfF4JH