The name pretty much says it all.

Reasons why this would be exactly good?

First off if it's a conversation with 3 people or more in main chat or in a forum you really can't move to messages on here. And especially if it's flooding the forum/main chat there's not much you can really DO.
This is however coming from someone who has multiple friends on here.

"But why not move to another platform?"
What if no one all has the same platform? For example discord? I know several people who don't have that or even WANT to touch it with a 60ft pole including myself.

Also not to mention if it's a private conversation between a few others and they have nowhere else to go but forums anyone can just pop in at any time.

A type of way these could work is that when your sending out a message you can put in and or select multiple people from your friends list or use IDs.

Similar to clans when your adding admins it could be for example like this. :
00001,00002,00003
So on so on.

But where would it go?
Well there still is plain empty spots in messages. So it could easily slide in right beside Saved ★. It having its own tab would be more convenient as well to separate singular messages from group chat messages.

How would it work/look?
I imagine that a user could be in 10 Groupchats at a time and can be removed or added into one. Or invited into one. The group chat could have its own name with a filter when naming it so it isn't named like a slur or something. And if your feeling a tad bit more fancy why not throw in the option to give the group chat a picture? The user who created said group chat also would have the option to change a few settings and also kick people out or add people in.

But what settings?
Well for starters.. Letting others change the name of the groupchat or the image of it. Either letting a select few do it or everyone. Or just themselves.

But how many people would be allowed in one group chat?
I'd imagine the max would be around 20. If not maybe 40. If there was no limit I imagine people would try to add everyone on site into a group chat and that would probably make the site slow down? But don't take my word I'm not coder.

I'm sure this has probably been suggested before.. and if it hasn't I'd be really surprised because messaging one person at a time is annoying and inconvenient if your trying to have a conversation with multiple people without flooding the main chat when other people are asking and or talking about the game. I have seen that on MULTIPLE occasions happen. One huge conversation in main chat. They have nowhere else to go so why blame them. Also this would help alot when people in main chat are asking for advice and are constantly ignored by other larger conversations! This happened to me before on my side account and its SO annoying.

What if I don't wanna use Groupchats?
Then there would be a button under account options/settings that would say "Allow Groupchats" and if it was set off then you would not be able to create, be added into, or invited to any Groupchats and anyone who would try would get something along the lines of "This player has Groupchats off!"

More the less that's all I gotta say. If you do no support please say why!