Is there any built in social aspect to SmugMug like 'friending' people or 'groups' on Flickr? I have friends who use SmugMug but I'd like an easy way to know when they post new photographs and I'd like to join groups of people who share similar photographic interests to me.
It seems to me that you are talking about social aspects with users who already have a SmugMug account. There are a few things which you should be aware of.
First of all, there are ways to indicate that a particular user is a friend or family. I believe you can share photos easily with them, among other things.
Secondly, you could join a community. There are a large variety of communities around, but they mostly focus on a particular type of image, and more specifically, networking among these types of images.
If you just want to follow a particular person, I recommend you pull their RSS feeds. There is a link at the bottom of all pages (Unless disabled). Mine looks like this. Simply plug it into an RSS reader, and you've got notification when a friend posts new pictures.
There sure is. You can use our friends and family feature and our communities feature. You can also easily share on Facebook and Twitter, Tumblr, Posterous, Wordpress (more to come!).