In order to add participants to a live study select the option to "Increase Places" from the blue Action menu.
The number you enter here should be the total number of places you want to be available for your study after increasing places, and not the number you want to increase by. The study's pricing will automatically re-calculate when you do this.
Note that you can only increase places for studies that are still active. If you want to collect more participants for a study that is already completed, you can duplicate the study and add a 'Previous Studies' screener: hc/en-gb/articles/360009094374-Preventing-certain-participants-from-accessing-your-study.