As long as this group doesn't mind that you keep coming back, you should jump at the opportunity, since it will make you a better and stronger rider. Experience only comes with miles and time in the saddle, so do different rides, but keep challenging yourself. If you feel like you're struggling to hang on with a group and are doing full rotations, don't spend much time at all on the front and save enough to get back on, otherwise you'll be cooked and quickly drop off the back. Most groups don't mind if you do short pulls, but they do mind if you slow down their pace.