The discussion on that page eventually outlined a solution, but it might be a little hard to pull together, and Discourse admin navigation has probably changed a bit.
This kind of thing is done by creating a simple Theme Component:
- Go to Admin → Appearance → Themes & components
- Click the Install button, and in the dialog choose “+ Create New”
- Name it something like “Groups page intro” and leave the Type selector on Component
- Click “Create”
- Select your theme(s) and click the Edit Code button.
On the CSS tab, paste:
.welcome-message {
display:none;
}
.groups-page .welcome-message {
display:block;
margin: 20px;
}
In the After Header tab, paste:
<div class="welcome-message">Put your welcome info and instructions here</div>
Save it, and refresh your Groups page – you should see your new text above the normal page contents.