Can I CC other people (or lists) on my posting?
When you post to the members' mailing list, members [at] origamiusa [dot] org should be in the "To" field and should be the only destination email address in your message.
This means that you cannot CC anyone on list postings. This is to save third parties from having their addresses exposed to the entire list without their consent. As a side effect, you cannot CC even yourself, nor can you post to multiple lists simultaneously.
And the members mailing list address needs to be in the "To" field, rather than the CC or BCC field, so that people replying to your message can easily reply to the list.
By default, when you hit "reply", the list's address will be placed in the "To" field of your mail. If you don't want many hundreds of people to read your post, check the "To" field before sending! (It's a fine habit to get into, anyway.)