There are a couple things you can look at. The first thing to consider is how many times light rays are allowed to bounce, which is determined by the diffuse depth parameter. Generally, the higher the diffuse depth the lighter your shadows will be.
If raising your diffuse depth doesn’t solve the problem, or if it adds too much to your render time, you can think about adding some fakes. One thing you could do is add a little luminance to the floor/ground material. Another option would be to increase the generate or receive GI parameters on materials that are reflecting/receiving light (material’s Illumination tab). Or you could do it the old fashioned way and add one or more invisible fill lights.