Mm, the second group wasn't too bad. I mean it was sort of uncalled for to laugh at you, yes, but there's no way I'm waiting for an LB... bosses hurt a lot and I'd rather they die right away.
Though it's better than when I first used LB. I main bard, which almost never calls for our LB since it's shared by healers. When I started leveling rogue, someone told me to LB in the chat during a dungeon and I actually didn't figure out they were talking to me until after the dungeon ended, haha.

I usually try to help people if they mention they're new. Just say you're new at the beginning of a dungeon, and one of two things will happen. "Oh okay, let's help you." or "Okay, bye," and a person will leave. Rarely do I find people outright ignoring newcomers. Don't be sad if people leave just because you're new; it's not worth it. Honestly when I started leveling healer, I would tell groups "first time healing this dungeon" which would, I'd hope, let people know I knew the dungeon, but not necessarily from that point of view.

Also definitely see if nice people you meet are on your server, if they are you can contact each other after the dungeon is over and maybe do more dungeons together.

Good luck