Installing additional outlet to the same line

Each outlet has 2 sets of positive and negative connectors plus one common ground.

Use the second set of connectors (unused) to link the second outlet to the first.

If unclear, call an electrician.

Meiwan,
Thank you, that was one of the options I considered. Wasn't sure every outlet has two sets of + and - connectors.
Question- how do i connect ground wire? I don't think there is a second ground connector.
The other option is to make two sets of jumpers from the power line, each connected to one of the two outlets.

Do it the Meiwan way. For the ground you could splice the ground wire into two, or daisy chain the ground from one recepticle to the other (using a short bare wire) so on the ground screw, you would have two wires feeding into it

You could also do your 'other option' whever you are comfortable with. No worries about voltage drops, I do not think it would be an issue

Be certain to connect the black(hot) wires to the brass colored screws and the white ones(neutral), to the nickle colored screws, so you don't reverse phase, between the outlets. Regarding the ground: just get some crimp-on, spade terminals and you can attach two wires(Romex ground and jumper) to your existing outlet's ground screw. (http://www.wiringproducts.com/contents/en-us/d150_fork_spade_terminals.html)