I always mention that I haven’t done it before if I do a new dungeon path, but if I put in a post on gw2lfg stating that I’m new to a dungeon, I rarely get an invite or people who want to join my group. I also had people leave me when I said I only did a path once.
I rather not join a group myself because I don’t know if they are ok with newbies. Most people on gw2lfg are looking for experienced players only btw, so I can understand why newbies sometimes don’t say they are new. Harder to find a group and afraid to get kicked.