By "modpack", do you mean 1 or 2 mods on top of a standard modpack like Unleashed, or do you mean a complete modpack built from the ground up ?
For the former, just drop the mod .zip or .jar file into the Unleashed mods folder and try to start it. If it complains about an ID conflict, you'll have to massage files in the config directory.
For the latter, you're better off not using the FTB launcher and using
Forge on top of a standard (vanilla) Minecraft install. You then assembled your modpack in the mods folder and for private distribution, zip/send/share the mods and config folders.
I have a private mod-pack on my 1.6.4 server and share with a friend using Dropbox. I have just the mods and config folder shared and we keep the server and our clients updated that way.