To start, click on the Portfolio tab on the left hand panel. Then, select the three dots in the top right of the screen and select Company metrics, as shown below.
Then, whichever specific metric you are wanting to customize, click the pencil on the far right of the metric, as shown below.
Next, toggle off the Track this metric for all my portfolio companies. Then, you can choose which companies you want to track with this metric, as shown below.
Finally, once you are finished adding/removing companies, click the blue save button at the bottom, as shown below.