Joe
What exactly are you after? May I suggest an alternate approach to finding things out about these organisations.
Have a rummage round the web for sites dealing with recent conflicts / ops.
Rather than getting the standard party line of how many countries and when they joined etc you may find out other aspects - what did countries bring to the party on various ops, how successful were they etc For instance - I bet the UN site won't tell you how the UN were almost kicked off the Korean Peninsula by the North Koreans in 1950/51 during the Korean War, and what saved them.
Just a suggestion that might add another 'applied' dimension to your research. And if you get stuck on hunting down what ops they've been involved in, try looking at the sites that sell / reference medals - they should have most of the recent (post-WW2) UN/NATO campaign medals to give you a starter as to where to look.
Good hunting