you can just say how you miss them and invite them for stuff, usually it works better that way, i think. if you literally say out of the blue you feel ignored and left out that may come out as offensive...

personally i made my peace thats just how most relationships are these days. people either dont have the time or they dont have the energy. when that bothers me i realize that a lot of the time im like that too, so it usually doesn't make sense to ask people for much more if i cant give it myself..