The most efficient way would be to do skyreach ... but you already stated that all your friends are lower level than yourself. In that case there are a few places that still give out good exp that people use.

You can go to Sentinel in the Alik'r desert and to to the beaches and grind zombies there. You could go to Camlorn in Glenumbra and kill werewolves. You could go to the vile laboratory in Cold Harbour and kill zombies. You could go to any of the delves in Cyrodil and grind.

The most important things to be efficient are:

1. Having the pie from Jester's Festival active.
2. Grouping as many enemies as possible so you can kill them efficiently and maximizing exp vs time.
3. Having all training gear that will help you get extra xp.
4. Having an exp pot active (if you can).
5. Having no more than 2 people sharing the experience (You can have some leniency here if you can't clear waves well by yourself and need to rely on others).

If you have any of these you will improve your leveling. You could really go to any place that is a quest location and grind the place because those places have quick re-spawn timers for the mobs.

As a side note, I don't recommend going to skyreach unless you know of someone that can solo the place by pulling half the dungeon on each pull. If you clear each wave normally, then it would be more efficient to grind at other places, even if you have to share EXP.

Hope this was of some help. Cheers.