Scoop's matching algorithm will always try to match you in a 3 person carpool by default. However, depending on your route's trip demand at your scheduled time, you may only be matched in a 2 person carpool.
Can I add another Rider from the carpool shortlist?
Unfortunately, the carpool shortlist is not available to Drivers who are already matched in an existing carpool. However, if you are an unmatched Driver, it is possible to match 1 or 2 Riders from the carpool shortlist.