Ok all,
I need some practical advise on feeding 20 or so people, cheaply, (...) DIY is a must.
Step 1) Figure out what resources are available at site. Working kitchen? Only electric outlets?
Step 2) Email everyone and find out what they are allergic (or intolerant) to, if anything. There's probably more vegetarians out there than you'd suspect, too.
Step 3) Can't continue 'til information is there.
Craft services is more than just the big meals, btw. Ongoing supplies of coffee, water, power snacks (nuts, raisins, m&m-things, paper products, fruits (dried & fresh) and more).
Keep a stack of paper cups (and a sharpie so people can write their names on their cup) so people can mix & match their own trail-mix combos into a container from bulk food supplies found at places such as Smart & Final.
I have several heating trays (some electric, some gel-flame) that I use for catering, if I'm cooking my own.
If this is for your 48 this weekend, do it all cold & prepped in advance. Or hassle your local sammich shop for platters. If you need hot foods, shanghai other people into making it. Unless you're the group's official caterer with no other time-consuming roles.
Catering is a fine art, right down to the timing. Good luck.