Actually when i do a search of members whom have registered between the dates of sept 1 to today, it gives me a list, when clicking on permissions it says not set (no) but they have all posted and been active. ?
The permission analyzer that Brogan mentioned will allow you to see exactly where all the not set (no) values are coming from, allowing you to determine which of those are incorrect so you can change them.
Permissions don't change on their own. If they were correct and people were posting, I can only suggest that you changed some setup somewhere.
Than it's most likely the new user permissions that are setting the no. But without running the permissions analyzer as suggested by everyone in this thread, or disabling addons to test, there is nothing we can do to help.
You need to look at more than just the final value, which clearly was going to be No because these users can't post. You need to look at the values for the user group(s) that are listed alongside this that combine to make up that final value.
I suggest you look at one of these users, look at the permissions, click on the Details tabs and show us a screenshot of the analysis if you're unsure about this, though hopefully it should be fairly clear when you look at this what the problem is.