I'm not sure if anything has changed but if you're talking about a private server modpack, distributed by FTB, I would assume that the information in
http://forum.feed-the-beast.com/threads/guidelines-for-private-server-mod-packs.9598/is still FTB's policy. Which means you will need to collect permissions. I'd suggest reading that thread and some of the related threads in that subforum.
Many popular mods have blanket permissions granted, but not all. I went through the process building a 1.5.x pack and by following the guidelines was approved on my first submission. I only had to contact a few mod authors and they were all friendly and amicable.
I'm not 100% certain that this is still the policy - I seem to recall SlowPoke or someone official stating that the process was being revamped with the 1.6 launcher, but I don't recall seeing any updated process announced.
Not trying to rain on anyone's parade, just trying to provide useful info. If anyone has a link to more up to date info I'd be grateful.
And for the OP, just a little friendly advice. Spend some time going through the configs and setting it up for how you and your friends like to play. I firmly believe that config settings are where you can really make a mod pack your own, and provide the subtle difference between just another collection of mods, and a real pack that has character.
Have fun!