I like your shape design, esp on the two guys. My main critique is that they've got same-face syndrome going on. Even with a minimal approach to drawing faces, you can still inject more uniqueness into the faces. Different jaw, eye, brow shape, etc. I do think noses are a very defining feature when it comes to telling faces apart, so that makes it a little tough, but you can still do a lot with the other features.
The girl feels pretty bland right now, honestly. Her haircut is cute, and I like her figure, but her outfit is very plain. It'd be nice if we could see some of her personality through her clothing design and pose. Curtis reads as cool and confident, sorta rocker dude, Jack reads as grumpy, maybe a jock type? But I can't get much from her at the moment.