The use of the comma in these cases is largely a matter of style as the presence or absence of the comma is of no real consequence with regard to the meaning and intelligbility. If you would like the reader to pause for a while and thus make too or either a little more emphatic, use a comma: They all know it, and their friends do[,] too. (Or: ...
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.