Ordinarily, these messages will not be seen or processed by a human. This is an alias that reaches the owner of the SIGs mailing list. There is always a real person on the other end of this address, so if you need to contact a human being, this is the address to use. Each mailing list has a policy for such things as who can join the list, or post messages, etc. SIG owners are given their own web interface to MailMan, so they can change the policy without our help.
Subscriptions are open to everyone.
Approval will only be required if the subscribing address does not match the mail headers of the subscription message. Anyone can post messages to the list, even if they are not members of the list. The list is not be moderated. Well here's what you have to do. Most importantly, you have to take responsibility for your list.