For me, I have reflectors which can be positioned (sometimes with help) so as to fill in the light on the subjects. There are some reasonable 5-in-1 options out there, I have a 43" version of one of them and it works very well, folding up to a pretty small package and giving lots of options for cooling or warming the light as needed.
If you don't have reflectors, but do have larger, bright, surfaces you use, then that too is an option. What it really boils down to is getting some of generally available light redirected to the place you want it and, for that, many reasonably reflective items will help a great deal if sufficiently large. Heck, it can even by some bristol board or shiny wrapping paper, I've done both.
Anyways, those are what I do and, to be honest, I prefer that sort of light over a flash anyways. Fill flash can harsh, especially when it is on camera, and so I prefer to avoid it.