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rbryan22Comment by jdannels: a few ideas that may help you.
Most lenses are not sharpest "wide open" which means with is its largest aperture. For your lens at the wide end it would be 28 mm at 2.8 or 70 mm at 4.0. Also zoom lenses "sweet spot" is generally in the middle of the zoom range and stopped down a couple stops, maybe 50 mm at 5.6-8 would give sharper results.
Another thing to consider to get sharper shots is your shutter speed. look at your focal length and multiply it by 1.5 to help you have a shutter speed that won't be affected by camera shake. For example at 70 mm*1.5=105. This means a minimum shutter speed of 1/100, 1/125 would be better.
So in this photo If you zoomed in to 70 mm your shutter speed is a little too slow, you are at f/4 which may be a little soft and accentuated with the slow shutter speed. I would recommend a little higher ISO, maybe 400 or more, I think your camera can handle noise well and would still gives you very nice results.
Another tip is to get closer to your subjects and try not to crop at all to give you more details to work with. May not help with your current lens issue but will help with prints and the less you crop the better you will hold details.
I hope this helps. If you are looking for a nice portrait lens the 50 mm 1.8D is a great buy but may not be the best for group portraits. It is one of the sharpest lenses I own. :)
Happy Shootin
Joe