It depends on what you are looking for and what the ultimate goal is. Blogging is less intimate, its more like public publishing. You do however have the option of creating private blogs which will require a visitor/member to enter a password or be personally invited by you. It's good to do some research because some sites requires a monthly fee if your private blog members are more than say 35. There are plenty of free blog groups. Ie blogger.com provides web layout templates to format the look of your webpage. Permissions can be set to make the site private. It also allows you to tag your blog and alert members that it may contain adult content, requiring the user to acknowledge that they're 18+ and you're not responsible for what they may stubble upon.
Yahoo and Google both have groups and both gives you an option to set your group page to private or public. Groups however leans more toward the side of being a large email forum. Mainly for communicating with members of your group and allowing file sharing,uploading etc. Groups, over blogs, gives you more of an option to maintain contacts and export
them to a file or email service. You can also send email alerts, notifications, updates to everyone in your group at one time. (Many of these sites have security measure to check if you are sending legitmate emails or spam)
Both option's security depends on the website and the type of security policy or measurements that website has. I say to all of my friends ANYTHING can be hacked into and NOTHING is truly deleted.
.
BTW I am going to checkout Windowslive.com now.