I probably took more pictures on full frame cameras with an EF 24-105 mm f / 4 L IS than any other lens. But for what you want to do in low light, it is not the best option. In f / 4, it's a bit too slow.
I would suggest a main lens of 24 mm, 28 mm or 35 mm. While cousins of 24mm f / 1.4 can be expensive because they are a challenge to design and produce well, there are more than a few affordable lenses of 24mm f / 2.8, 28mm f / 1.8 or 35mm f / 2 in the market. Compared with its previous culture body, 24 mm would give the same field of vision (FoV) with the 5D Mark III as 15 mm in its Rebel T2i / 550D. 28 mm in FF translates to 18 mm in the 550D, and 35 mm in FF is the same FoV as 22 mm in a 1.6X culture body.
Another great option would be the EF 16-35mm f / 2.8 L III, but it is even more expensive than the EF 24-70mm f / 2.8 L II. The previous versions of the 16-35 / 2.8 L are not optically as good as the "III", or the more economical EF 16-35mm f / 4 L IS (if you can do without the maximum aperture of f / 2.8). For larger landscapes with an FF camera, which includes cityscapes at night, the 16-35 / 4 L is a large lens. But I think you will not find it fast enough for group pictures in low light.
If your budget does not allow the EF 24-70 mm f / 2.8 L II and is fully concentrated in a zoom lens, my advice would be to go with the less expensive Tamron SP 24-70 mm f / 2.8 Di VC. It is sharper than the original EF of 24-70 mm f / 2.8 L (which I still use because it gives me what I need for a 24-70 / 2.8), but not as sharp as the new "II" introduced in 2012. The Tamron also offers something that Canon does not offer: Vibration Compensation, which is Tamron's term for & # 39; Image Stabilization & # 39 ;. I have several friends who have the previous version of Tamron, and are very happy with it. Tamron just released an update & # 39; G2 & # 39; (& # 39; Generation Two & # 39;) a few months ago that added the ability to update the firmware and calibrate the AF using the USB port & # 39; TAP-In & # 39; of Tamron. it is currently selling for a little more than it was before it was replaced, but it is still several hundred dollars cheaper than the Canon EF 24-70mm f / 2.8 L II.