What is a tag?
Tags are a way to group users together in an ad hoc manner.
Who can create a tag?
Anyone but if you make something inappropriate prepare to be banned.
I made a tag how do I add people to it?
That's the neat part ... you don't. Tags are for others to join you have no control over a tag once it is created. All tags are opt in.
So why join a tag?
You can make a tag with your friends and find all of your scores together on a single page for quick reference.
Are there any special tags?
Yes for now there is one. Beta Testers If you join this tag you are opting in to beta releases of the site. You will get to see new features before anyone else but you will also see more bugs.
Okay sounds great how do create or join a tag?
Head to your athlete page scroll to the bottom and hit the update profile button. You should see a section for modifying tags.
Wow this is great but I really want to control the grouping of people for quick reference.
Not a question but I'll still answer. What you are describing is a Premium Feature on the site. Specifically you want to look at the Side by Side feature. Tags are a free version of Side by Side to support groups, while the premium version gives you more control and private insights to your friends / rivals.