Sure you can join!
Currently @aqua03 drew a sketch which I lined. You can start by coloing that.
Then I need to sketch something for you to line and give to aqua to color
And you need to sketch something that Aqua will line and I color.
You can do your sketch before the color I don't mind. I've just personally been waiting for a 3rd person before I made my sketch. I don't think there are any themes or anything, just sketch whatever.
Also here is the lineart that you can color