How do I email a News / Blog post to my members?

Open the post, publish it, then go to its Config tab and click Send to members under Email This Post to Members. It goes to the members of every site the post shows on (its Where It Shows setting), and a person who joined two of your sites gets one copy. A paid post goes only to paying subscribers.

Sending is always your choice: publishing a post never emails anyone on its own. The button waits until the post is published, because the email carries the published version. Config also shows when a post was last sent and to how many people, so you never send twice by accident.