Influence, in-fluence. Fluency. I guess fluency in language, emotional literacy or artistic endeavour helps one influence people, i.e. flow into them.
The word 'concentration' formed of the words con from the Latin for 'with, together' and centre (the Latin centrum) is self-explanatory enough: to bring or come to a common centre.
To concentrate is to centre oneself for the purposes of mental focus and a con-centration of people or things means they are moving in the same physical location, i.e occupy the same centre, as in a concentration camp.