The best thing you can do for them might be either occasional feedings of shell-in nuts(not fed by hand, but left behind for them - feel free to observe from a distance though!), or setting up a small feeding spot in a squirrel safe and friendly area, away from humans to keep them from being a nuisance and potentially trapped and killed, as well as a spot safe from predators.

As another commenter mentioned, it's not really a good idea to socialize with wild squirrels very much because they can then become a nuisance to other people and animal control will get involved. Plus squirrels can be vectors of disease to the same extent that rats and mice are, and they're more prone than either to have rabies.