You want to provide your users with "default groups", so they get to know Grape faster? No problem!
Every Grape organisation has 3 default groups to start with: development, general and off-topic. You can rename them, of course. You can even delete one or two of them or create more default groups for your users.
For this, click on the gear at the top left and go to "Manage members"
Click on "Teams" in the middle section next to "Active members". Here you can choose which users are in which groups per default.
The very first team is always for "all users" with the 3 mentioned default channels pre-set. Click on "Edit" to make changes.
In the following view you get to choose the conversations all users will be added to.
Important: You can only select public groups. Inviting all users to private groups, wouldn't make much sense anyway.
By clicking "update" all changes are made and all users are added to these groups. Users cannot leave these groups, but always have the option to mute them in case it gets to loud in there (see in notification settings).