The only forum rule that I found that hints about not advertising groups is this one:
When you click the flag button it says this under the spam button:
This post is an advertisement, or vandalism. It is not useful or relevant to the current topic.
With forum rules you need to use some common sense because Roblox can’t cover every single edge case within the forum rules thread. I am certain advertisement threads of any kind aren’t allowed on the forum because they count as spam and will bring a lot of unnecessary noise for the forum.
As there are no categories on the forum that members can access that support these sorts threads I wouldn’t attempt to post group advertisement threads.