I would ask when tucking in behind the group - or behind an individual rider - if its okay for me to sit on, then after riding a bit and getting accustomed to their pace and riding ablilities, I'd offer to take a pull. That's if I thought I could hold the same pace without absolutely dying! If not, and as long as they don't seem to mind, I'd hang on and then thank them profusely at the end of the pull.
Pace lines can be tricky. You want to know what you are doing and be comfortable riding in them before you jump in with people you don't know. If I ever get in a line and then can see that there are inexperienced people ahead of me, I either move up, or get out. Too many bad things can happen. I've seen numerous wipe-outs. All I'm saying is -- just be careful! Practice paceline riding with experienced riders, don't jump on with just anyone, and try and keep your pace as predictable as possible.
Time is a companion that goes with us on a journey. It reminds us to cherish each moment, because it will never come again. What we leave behind is not as important as how we have lived." Captain Jean Luc Picard