We've implemented a rate-limit on the number of posts a new user/site can make to groups to limit new users spamming groups with irrelevant posts.

The limit is currently set to 3 times the number of weeks since the site was created (for example: if your site was created less than 7 days ago, the total number of posts to any group that you can make per day will be 3; if you joined 8 weeks ago, you can post 24 posts to groups per day).

Posts to your own site/group are unrestricted regardless of the age of your account.