There are a couple ways I have found to do this.
If you are using desktop there is an option under the 'For Owners' tab that is labeled 'My Watch List' that shows everybody in alphabetical order - highlight a name there and click the button at the bottom of the page.
Also, if you click on a name from the 'Player Stats' tab it opens options at the top of that specific player's page. One of those is 'Add To Watch List'
If you are using the phone App you can search under 'Find A Player' on the left side then select a name from the list, then click the arrow at the top of the page(next to the map) and that will open up options for Add,Draft, and Add to Watch List