[Chibi]

If you mean "modified" as in set-up-in-game, you'll need to send your friends your saved variables, not the addons themselves (though obviously they'd need the addons installed to use the saved variables)

You can find your saved variables in the WoWDir/WTF/ACCOUNT/*Account name*/ folder, though depending on the addon it may either be saved per account (WoWDir/WTF/ACCOUNT/*Account name*/SavedVariables) or per character (WoWDir/WTF/ACCOUNT/*Account name*/*Server Name*/*Character Name*/SavedVariables)

You'll have to take the .lua file in there named after the addon, and have your friend put it into the same folder on their computer. (Under their account/server/character name, obviously)

If you mean modified as in opened-up-the-addon's-lua-file-and-changed-things then you'd need the addon author's permission to upload it here (and under some licences, to even modify it for personal use, though that's okay in most cases) Or depending on the addon's licence, you may be free to distribute it directly to your friends as long as you credit the original author.