You can protect your whole Posterous site with a password. See the Privacy section by clicking Settings in the action dropdown from the Spaces section.

Be sure to save your settings by clicking Save Settings at the bottom of your page.
You can also set a seperate password for your Space. After you set a password, everyone visiting your site for the first time will be asked to input the password you selected to view the Space. Your visitors can set their browser to remember the password for subsequent visits. If you would like to reset this automatic site access

In the People tab, you can manage who you’d like to receive emails containing your new posts, who you’d like to contribute to your Space and who has complete control over your Space.

Subscribers are people who you want to receive your new posts via email. Adding someone as a subscriber is an effective way to ensure your latest posts are viewed by people who don’t visit your Space or group every day.

Contributors are people authorized to post to your Space or group. Adding contributors enables you to have multiple authors for your Space. There are two ways a

With Posterous Spaces, you control exactly who can see the contents of your website. Share with the world or with a select few – it’s up to you.

Public
Public Spaces can be seen by anyone who visits the Space’s URL.
Ideal for: Bloggers, portfolios, business sites

Only those you invite
You can invite others to be members and followers to your Space. Members can contribute to your Space and followers receiving updates in their email and in the dashboard reader.
Ideal for: Family photos, private communications, clubs, groups, classrooms

Those who know the password
You can password protect