The issue is not how to find a group or clan. It's how extreme the disadvantage to play solo currently is.
Firstly solo play do not need to be more rewarding than multiplayer in any way.
There is a huge gap between making solo play more rewarding than multiplayer, and where this balance sits today.
Inheritly playing any games with friends offer a better experience than playing it by yourself for most players.
This alone is such a big factor, that multiplayer do not really need any other advantages or incentives to keep multiplayer the prefer mode in general.
But that's where Blizzard has fucked it up so badly, by adding way more incentives / rewards than needed to multiplayer.
XP gains, and loot sharing is the obvious carrots, but how a group simplifies the game when it starts to get difficult solo is what Blizzard should really consider.
The element of having 3 other players being targetted aswell, how mobs dont scale properly with number of players, how paradoxal support or zdps specs role is in a hack'n'slash game.
Blizzard should also really consider the reasons why some choose to play solo, why some actually enjoy playing the game as it was meant to be.
The ones who do not care about cheats/huds, "meta" or fanboying some youtubers / streamers.
Having the option to play solo when your friends is not available, with any spec or class you want.
Instead of having to get carried as a support since thats what they need for their circle-meta-jerk.
If just mutliplayer / soloplayer was balanced somewhat in regards of difficulty firstly,
it wouldn't feel as crippling to play solo, even if it yields abit less xp and loot.