-으려고 is used to express the purpose or intention of an action. It has the same meaning as "in order to do some action." (Source: 서강 한국어 2A handbook. P9)
The New Ace KOR-ENG Dictionary defines -러 as meaning "for the purpose of; in order to."
What I usually hear, think, and what I just verified with my wife is that the key difference is "Intention." So with this in mind:
"친구를 만나러 여기로 왔어" would translate as "I came here to meet my friend."
"친구를 만나려고 여기로 왔어" would translate as "I came here to try to meet my friend" or "I came here intending to meet my friend."
Comments welcomed on my answer. :)