Readings & Meanings:
To fit, to be together
合 is used in pretty much everything. Here are just a few:
If we decide to hire you, we will let you know via phone in a few days.
I feel sick to my stomach. (lit., my stomach's condition is bad)
I got along so well with the guy sitting next to me at the pub that we ended up drinking together till dawn.
(copied directly from http://home.alc.co.jp/db/owa/s_kaydic?num_in=8&ctg_in=2)
If you want to say to your friend "Let's meet," this is not the あうyou're looking for.
My thanks to http://www.jlptstudy.com/2/index.html and http://www.thejapanesepage.com/kanji/kanji/a.htm for great JLPT sites with better navigation than I can manage here. Also to http://www.alc.co.jp/ for a great translation of that 4-kanji idiom example.