They go by their respective nicknames. If you don't know them, shame on you.

Marauders' Group Chat
*Moony joined the chat*
*Padfoot joined the chat*
Padfoot- Yo, was sup?
Moony- Doing homework, but not really since I'm messaging you.
Padfoot- Uhh, ok. Hru?
Moony- Fine, hbu?
Padfoot- Smoking on the Astronomy Tower.
Moony- Why?
Padfoot- So I can threaten to jump off if Professor McGonagall tries to give me detention.
Moony- No, I meant why are you telling me? I can give you detention 'cause I'm a Prefect.
Padfoot- You wouldn't?
Moony- Why not?
Padfoot- I have a huge supply of dog food I can shove in your mouth as you sleep if I want to.
Moony- You wouldn't.
Padfoot- That's what you said yesterday. And I proved you wrong.
Moony- I'm leaving. My life has no meaning.
Padfoot- Okay, see you tonight.
Moony- OMG, I am so excited for the break in plan for Honeydukes. Imagine McGonagall's face when we give her a huge treat bag. Also, free supply of chocolate! Yay!
Padfoot- Ily, bye :)
Moony- Wait did you just-
*Padfoot has left the chat*
Moony- And, he's gone.
*Moony has left the chat*