As others have said, you probably discovered the limits of your 18-200mm Nikon lens in terms of edge sharpness.
It's hard for any lens to maintain sharpness all the way to the edges and corners when used at the widest aperture. It's even harder when it is a zoom lens. It's harder than that when it is a zoom lens with a focal length range of 18-200mm! The best zoom lenses rarely exceed a 3:1 ratio from longest to widest focal length. Many prime lenses will equal or even exceed the corner and edge sharpness of those premium zooms at 1/5 the price or less. Pretty much any lens is sharper stopped down a stop or two that when used wide open.
When shooting large groups in marginal light camera stability is a major factor. If you use a tripod, a cable release, and mirror lockup it will allow you to use slower shutter times than would otherwise be the case. Because each person in the frame is such a small percentage of the total frame, the same movement that would be very noticeable blur in a tight head shot of a single individual is not detectable in a group of 100. The angular size of each person's head in a group of 100 can be smaller than the pupil of an eye in a solo headshot! Thus if the camera is stable, vibration free, and contributes no blur to the entire frame then the size of your subjects means they probably won't either.
It's not exactly a family reunion, but it is a group of around 100 people. They're standing to recognize applause at the end of a judged musical performance. The students are not necessarily aware that they're also posing for a photo! The reason a zoom lens was used was due to the fact I shot 33 groups over a two day period that ranged in size from 25-30 to over 100. The zoom lens allowed me the flexibility to frame each group optimally from the fixed shooting position. Due to the fact that it wasn't a "posed" shot I took several successive frames of each group and used the one in which the group looked best overall.
Shot from the elevated projection booth at the rear of a 1,500 seat auditorium with the existing stage lighting which wasn't very bright.
Tripod mounted Canon EOS 5D Mark II with EF 24-70mm f/2.8 L @ 60mm. ISO 500, f/3.5, 1/80 second. Mirror lockup and wired cable release. Manually pre-focused on a point 1/3 of the way between the front and back of stage using magnified Live View.