There is no rule about certain number of posts. Anyone that expresses a wish to contribute to Sat Universe can be given attachment permissions based on admin's decission or other team members suggestion.
No, that's not right. Once you are given permissions to post files you can post any time you wish. I am not talking about "permission" as in consent but rather modification to your status in board's database. Off course permissions can also be taken away if someone abuses them to post inappropriate material.