You probably can't. Private companies are picky about who they let on their cap tables. That's what makes them "private".
The first thing people bring up when people ask this question is ROFR, but I think that may get your hopes up. ROFR suggests that the issuer might be forced to match whatever price you find for your shares in the market. Put aside for a second how hard it will be to get a price for those shares.
What's (I think?) a little more likely to be a problem for you than ROFR is that your shareholder agreement probably more or less prohibits unauthorized transfers of company stock. Boilerplate agreements have clauses requiring board approval for transfers.†
Finally: it's awfully hard to sell shares in a business without being able to disclose the numbers for the business. But you probably won't have the applicable numbers: revenue, projections, expenses.
The stories you hear about private shares being sold in secondary markets usually involve gigantic runaway successes, like Twitter. A company having raised 8MM absolutely does not mean your shares are worth anything on paper; the money the company took from investors was pledged to growing the company as a condition of the investment.
† (good: http://www.nycbar.org/pdf/report/uploads/20071830-TheEnforce... )