To expand a bit on what Gedstar said, we don't know you from the proverbial hole-in-the-ground. We don't know if you are 8 or 80 years old, we don't know if you are a doper or straight as an arrow, we don't your other likes and dislikes, the importance of humor or nostalgia to you, and certainly don't know any of these things about your friend. In addition, we don't know if you are going to have this printed and framed (...if so, how large...) or viewed on a computer, we don't know your level of skill with Photoshop, etc. etc. etc.
So give us all the above info + maybe some examples of collages you like, and we will be happy to try to help you out. Without it, we are shooting completely in the dark, and most likely like 99% of what we would suggest, your or your friend would instantly hate because we don't know even the most basic things about you.
Dear gedstar and Tom Mann
Yes im sorry i didn't give you enough info
This Card is a collage of photos for my sister and her friend, they are 26 years , my sister want to make a card with their photos and to be printed and framed in a small frame and to be posted on Facebook consider my self as intermediate on Photoshop CS6
Another possible idea if it interests you... the video that Gedstar provided shows several photos essentially tiled into one overall collage. You say you have some Photoshop experience, so another approach is to individually cut out your sister and her friends from each of their individual photos and paste them all into a common background, somewhat along the lines of the crowd of people on the Beatles' Sgt. Pepper album cover.