Yes, the grind is strong. A long time ago, the year it came out actually. I think the problem with getting four people together though is that the players levels will differ drastically, unless someone's levelling up a new warframe or something.
I was doing a boss run with two random high-level people, and I was quickly left behind when they just rushed through the enemies and left me standing there with my pitiful shield and pea shooter. When I caught up to them, they had just killed the boss and we headed to the extraction point. I don't think I've seen more than a few new players, most people are levels 6 and above with potatoed Prime warframes and weapons. I'm still playing on my newbie-frame and cheap weapons. One of these days I'll get the three control modules I need for a Sentinel and the Rhino warframe...
Maybe.
And levels don't really mean anything, other than the amount someone has been grinding the shit out of everything (which in turn of course correlates to extremely well-modded weapons and frames, but, yeah...).
For example, one or both of these guys are "higher" than me, but I still owned the battlefield:
Effective long-range CC is my friend.