Some mod authors don't want their mods published in public modpacks, the biggest example I can think of being Optifine. Sites like curse and technic most likely won't let you publish a modpack if you include a mod that you're not allowed to have, or will take it down if they're asked.
If you're just distributing it to your friends you should probably be ok. It depends on what license the individual mod author uses, but most generally accept that you can give it around within your group of friends. Even if one says you can't, there's really not much they can do.
As long as you keep it private and don't make any money off it you'll be fine.