I would consider glass or mirror for the side tables, with one or more having a nesting function given your love of entertaining. For additional seating, rather than cluttering the space, consider something like Cubista from Resource Furniture ( http://www.resourcefurniture.com/). There are multiple upholstery options available and you could use it as an ottoman or side table when not opened up to create seating for an additional 5 people.

Adding some colour to the walls, even just one as an accent, would look great! And some artwork or a large mirror, as well. If you're not married to that area rug, I think something a little larger would really ground the space and pull it together.

Nice room! I would start with painting the walls a rusty red to pick up those colors in the rug. The contrast with your furniture would be stunning. A round coffee table would be nice, placed in the center of the sofa and chairs. I like the one pictures as it is on wheels and could be moved easily to accommodate extra seating. I would not put any other permanent seating in the room, but instead get some nice folding chairs that could be used when large groups are over. Also a new floor lamp. The one pictured is from Lamps Plus http://www.lampsplus.com/products/blue-and-brown-floral-walnut-rectangle-floor-lamp__t1391-t6528.html. You can also purchase the shade separately to be used on a new table lamp.

The design and color on the rug don't seem to go with your modern gray and blue look. Maybe a solid color? I can't tell what your flooring is but I think a solid black rug would look very modern and go with your more modern looking seating.

For more seating maybe try some floor pillows?

Yes, replace the wood tables with crome and glass (not all but coffee table and one end table) Bring the little wood end table out of the corner and up to the edge of the sofa so it's useful. End table height should be almost chair arm height-slightly lower or slightly higher.

I'd change the rug to something like the rug pictures I linked. I'd change the lighting to a more modern one too (one that doesnt need a table) and then potted plants for the corners instead of decorative ones in a vase and a moremodern coffee table.

plant

You could replace your rug with something larger that introduces other colors. Your rug is nice but a bit neutral.

For extra seating a couple of these leather butterfly chairs

Instead of putting end table back against the walls bring them forward to almost the front edge of your chairs/sofa. Get a neutral shade instead of blue for the floor lamp.

Add some colorful art above sofa.

Love the armless sofa-bench! Try using it in the center as a "cross-linking" piece between the sofa and chairs. That way people can sit on both sides of it, conversing with the sofa people and the chair people. Put the chairs where it is and use the side table from the left of the pic, or the one with flowers, between them. You'll need one huge rug to anchor the space or two smaller, and very similar ones, to put on either side of the blue sofa-bench.
The other two side tables are too tall and too short for your seating pieces, so ditch them. Use your coffee table between the sofa and armless sofa (18" away from each of them.) Here's a chart for judging table and lamp heights as you'll need to work on that for balance.
Please try this arrangement, and post by commenting and using the Attach Images button to add pix of the update. We'll tweak it on out from there.

A pair of these chairs with this adjustable round table for drinks might make a good filler in the end or a corner of the room we don't see.

loved your existing sofas. few points which you can incorporate are.

1. change the rug, probably something colourful.

2. change the wall colour, or use some grey tones.

3. have some table lamps.

4. need to add throw cushions

5. need to add something in the wall, eg art pieces, paintings etc

the earlier comments had them all too

go for geometric painting on the wall. it should be blue lines on grey background or grey lines on blue background