You should be able to group with others at any level. I remember grouping with a buddy in tutorial zone during beta. I did it through my friends list. I think for randoms you have to right click their target portrait, although i'm not too sure. Now I only grouped up for dungeons with my guild which was only a handful of times. I mostly went solo because they were so slow. I think first group dungeon is at 15 and the next one is at 20. There's also 1-5 man housing dungeons that scale to your level. I've got the level 20 and 30 housing dungeons. The dungeons were pretty interesting for the most part. You're able to choose how you want to tackle the situation. I gotta wait for my guildmates to level a bit more for the next dungeon. I don't remember if it was 25 or 28.
I personally think the quests past level 15 are a lot more interesting but as you get towards the end around 40 it gets repetitive and frustrating(CORRUPTION IS BULLSHIT WHY DOES THIS MECHANIC EXIST). 1-14 is sooo boring. It's essentially a long ass tutorial. I'm gonna be making a spellslinger alt soon and I dread the first zones.