In my experience, a clan gets filled up when social media is involved and under the right circumstances, like clan owners giving up their leadership and clan-jumping, plenty of players who get bored with their disorganized clans, in-game as well. Before I involved social media, I couldn’t get to fill my clan (Fury Guardians) because some “elite” clans here were stealing so many potential members on the forums by making 3rd and 4th subclans and promising people stuff. After I did, there was an explosion of people and we’re now close to getting a lvl 7 chest.
We have collected a lot of Alpha points so recruiting is fairly easy. Because the more points we collect, the more serious our clan seems to outsiders, who consider the clan as an option. And not to mention that you attract the right kind of players too.
Clan chat wasn’t always the most active, but it is now that I’ve gathered a lot of good members. Not the best chat, mind you, it’s all stiff and such, but it’s functional for us. We talk a lot there, and although some messages get “lost”, it’s fun and we can talk for hours.
Have made a Discord server as well, to be allowed to talk more and share stuff we can’t in-game, like files and screenshots, and have some organization about it too, and now am struggling to get my members to connect there.