Fake the dropbox referrals to get more spaces

If it's hard for you to convince your friends to sign up and install dropbox so that you can gain more spaces as they're your referrals, you can create different mail accounts yourself and send invites to those emails. But this would not work if you're using the same computer to install dropbox and sign in with more than one account. Dropbox identifies your machine based on Mac address or Network Card's address. The addresses are universally unique.

The solution is using virtual machine, in which you can set your own Mac address for the Guest OS. In my case, I was using VirtualBox and followed the steps below.
1. (optional) If you're already in the Guest OS and the dropbox was already installed and linked, go to Settings of Dropbox then Accounts tab and click on "Unlink this computer" button. Shutdown the virtual machine.
2. Select on the virtual machine and go to Settings then change the Mac address under Network section.
3. Start the virtual machine
4. Sign into the invited email account and click on the Invite button in the email message
5. You'd get redirected to the sign up page of Dropbox. Follow the instructions to sign up and then the Dropbox program would automatically get downloaded at the end.
6. Install the Dropbox program, start it, and sign in with the invited email. You're done!

Problem 1

I was going to create 10 Gmail accounts as my referrals but a phone number could be used to verify only three accounts when signing up and I had only one phone.

The simple solution is using different mail services such as outllook.com.

Problem 2

In step 5 I noticed that I got the error message "Please enter the CAPTCHA resopnse" when I clicked on the submit button when it's the third time I've signed up on dropbox. I think it's a Dropbox's bug. It tried to prevent robotic spammers but it didn't provide a place to enter the captcha.

I found a workaround to bypass the captcha. I used VPN client to change my IP address so Dropbox couldn't track how many times I signed up anymore.