There are several different ways we can say that, but the most common of them sounds like this: "Приятно познакомиться", which is used in all cases: in a formal of informal way, to one or more people. Among friends, we usually say: "Очень приятно", and reply to it: "Взаимно". You can also say: "(Очень) рада знакомству" in an informal situation (it still sounds a little bit informality, but just a little).
It was pleased to meet you - было очень приятно познакомиться.