Post a message in General, with the name of the thread as the name of the group/organization.
Then in the main post put the purpose, etc.
As the group progresses you could put members on the main post.
When the group is doing well a wiki page is worth looking into.
Don't ask me how to make a wiki page, get someone else to, I CBA.

@cctvdude99
to get a wiki page just edit the wiki