Amanda posted on the forums explaining her connection to the young (amateur) actress playing Julia, and how she came to join Lonelygirl15:
The character Julia is played by my sister, Shannon Solomon. I am posting this now because I would like you to understand the context in which these videos were filmed, how they were filmed, and why.
A few months ago, I visited Shannon for the weekend at her home in Texas. Looking to bond with my little sister, I decided to make a few videos with her. We spent a Saturday afternoon laughing, making wardrobe changes, and spontaneously making the videos you have seen Julia post.
Shannon is not an actress and there were no scripts. We were two sisters having a laugh and being creative together. These were intended to be silly fan videos that would fade away into the Youtube landscape. We never intended to have them viewed in such a widespread manner.
Then, as Miles recently mentioned, we had a casting problem and an actress fell through at the last moment (just a few hours before filming). We all thought the videos Shannon had made were cute and decided to post hers instead. Shannon is a huge fan of the series and was thrilled to make her own contribution.
Shannon is 14 years-old and is in the 8th grade. She is a cheer leader, plays softball and volleyball, and I am very proud of her.
Shannon's a tough kid... her idol is Lucy
As for her continuing in the series, you'll just have to stay tuned and see what happens next.
ED: I used the word "(amateur) actress" to describe Shannon, because she is not a professional actress, but is playing a role. This was not meant to be derogatory or to contradict Amanda's statement that "Shannon is not an actress."