It'll be tough to light up 70 people with ambient light. It's actually easier in the dark. But in low light, but still light, ambient it's going to be hard to get an even spread. No matter what light you get, it's going to be harder and harder. Personally I think I would use speedlights myself, simply because I can employee 6 of them right away and control them remotely and just toss them on stands and use them bare and just fill the whole group from several angles and cross them so that the shadows are not hard. I have a 600ws portable strobe (Rovelight), but the only way I'd think to use it to nicely light that many people would be to create a large modifier to turn into my singular source and back it up as much as I can, like a reflective 7 foot parabolic and push it back until the light is spilling around the 70 people. In late evening it's dark enough that it should still be able to light them up from a distance with a large modifier.
The easiest way to do it is going to be to group your people up so that they take up less surface area. Rows upon rows, dense pack, rising up (think bleachers). Then you simply just raise a light high enough and at a good position to drop fill on each face. 70 people is a tough group if you don't have a mechanical set of steps or bleachers to use. Even having rows of 10 people, sitting, then kneeling, then crouching, then standing, and even behind them, the rest jumping last minute, that's only 60 people covered... lol. Just kidding about the jumping. But hey, you could...
Yea, instead of going brute force, I'd go speedlites. Lots of them. I have a fleet of 6 for these kinds of things. I use basic manual flashes (Yongnuo 560 III's with a TX560 controller, so I control them in groups and can change their output power from the controller). I have a bunch of 9 foot stands. I would just fan my lights out and just cross angles so that shadow is met with more light and just light everyone up. Bare flash. All of them on the same plane so that output distance is consistent from light to light to subject and out in front of the group. I use a portable stepping stool to get up off the ground a few feet, but you can also back a car up and stand on the trunk or something.
Also, with ambient light still present, not completely dark yet, you're mixing cold flash temps to really warm ambient temps, so it will look "flashed." You could gel some CTO's to get them closer. Or you could do it in post, if you're proficient in that. Personally I just gel my flashes. I'd probably use 1/2 CTO's (I buy 24x20" sheets of Lee filter CTO for about $7 on Amazon and cut my own strips enough to do two dozen speedlites, and my strobe, with left overs). I use double sided velcro tape to put it on my lights and hold it there.