Let's say you want to follow a particular group of individuals and see their posts in the community, but you don't really want to be emailed every time they post. Or, maybe you do want to be emailed. Or, you just want to separate the people in one geographical region from another that you follow... This is a two step process.
1. Create an Activity Stream.
2. Browse for people in the location you want or with some other profile characteristic, and then add those people to the created Activity Stream. (How to Use the Filter Function when Browsing for People)
How to create a custom Activity Stream: