There are loads of companies who do tours, which you can join onto - like previously mentioned Explore, Exodus and also Imaginative Traveller, Journey Latin America, Travelbag, Dragoman.
You dont say what age group you are, but having done a couple of those group holidays, I have found it's mostly single girls in their 30s, with a smattering of couples, the odd single guy, and a few older people (60's/70s).
Its a great way to see a part of the world when your friends have no time/inclincation/funds, and all the hard work of organising hotels and transport etc is done for you.
On the down side, your independence is compromised since you are in a group, you may not like the group dynamics, you share a room (unless you pay a suplement) and tend to have little choice of when/where to eat (unless you branch off from the group to eat).
Travelling alone you are completely independent and free, and it's generally very easy to meet people. It can be a lonely though, so depends on ones individual personality as to whether you would enjoy it.
Enjoy your hols!