You have to go to Bungie.net
Then sign in.
Once you've signed in, on the top left corner you see a destiny symbol thing, click on it and it'll have a list of things you can click on.
Click on the one that says “Clan" then once it loads, at the very bottom there will be a place you can write stuff on. Write whatever you want on it then click submit on the right of the text box.
After that you should be able to just use
group chat for your clan using the app.
Hope this helps anyone that was having trouble with this as well :)
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If it doesn't work, once you're at the clan page. Click on playstation clan, or Xbox clan, depending on your console. It says it at the top of your clan name. After that an option that says “Group Wall" will appear on the left of where it says “Group Forum" click on “Group Wall" Then once you're at that page, at the very bottom will be a text box, write anything down on it then click submit. Once it posts to your wall you will be able to go into the group chat on the destiny app.