Since a population can contain participants from various studies (even across teams), we've built in a labeling feature to allow you to organize your populations.
You can apply multiple labels to a participant, and you're welcome to create as many labels as you want. Color-coding provides some additional help in differentiating one label from another.
To create a new label, first select at least one participant you want to assign that label to. You'll see a button called "Add Labels" appear towards the bottom of the list. Click that button. Start typing in the new label and click on the option to create a new one. Then, click "Add".
You can remove labels from any participant within their profile. Click the small blue person icon next to their name from your list. This will take you to their unique profile. Once there, hove over the tag you want to remove. A remove tag will appear; click that. Here's a short video on how to do this.