Haven't checked it out in a while~...last time I did it was kinda active on the high-end side, so up until that content you would be soloing basically. Devs are now releasing new classes from time to time to somewhat shake community a bit, but I'm not playing/following the game anymore, so it's hard for me to say whether lower-lvls are active or not, but the game itself seems to be getting new content, so it's still active in that sense. Season 1 (all soloable, and with newer Seasons gear should be even easier to) and early Season 2 content was pretty good in this game, but after that devs made it unrealistically-grindy, where you would run the same Bosses for months without a rare drops that you need to make best gear, and the next content requires it (unless you have friends that will power-carry you thousands of times on the next tier Bosses, no one will want you in party with Attack a lot lower than theirs), which in its turn was also unrealistically-grindy, and so on and so forth.
As far as I know people still play high-end content. If you don't have a group of friends to play in it with (and it's way more enjoyable in that way), then probably you'll be on your own for the most part, until you'll reach that content. I have no idea which catching-up mechanics devs have implemented, while I haven't played the game, but I guess there should be some. If you'll start playing this game then my advice would be to save up money and buy like a 2nd or 3rd tier gear off the market, should be way easier than grinding mats for those I guess, unless they're given out for free by devs now