If you go for a two person pre-pitched and then your friends decided to go along as well, you can just arrive up and ask to be placed next to each other (if there is space, of course). We've done that for the last 2 years with a mix of pre-pitched 2 and 4-person tents.
I can't answer how it works for the space booking.
They’re one of these acts that probably aren’t as big in the UK as they used to be, but have enough fans to sell out an arena run, or a stadium. Not sure how many younger Glasto goers would know more than a couple of songs, if that. Probably should have done the Pyramid 20+ years ago, when they still had some relevancy