Data mining from Facebook groups -- 3

We want to build an email database from the members of certain Facebook groups through an automated process. The process will involve extracting members from Facebook groups & then finding their emails. The total members in the groups are about 70k members. We would require a database of at least 28k valid & unique emails (i.e. 40% success rate) to built by extracting information against the 70k member data.

All the relevant tools required during this project will be provided to the freelancers from company’s expense.
Please note:- We don't want freelancers to scrape the information manually.