Including additional authors to reduce time
As some comments above indicate, there is an important rule of thumb from the field of software engineering, which is Brooks' Law.

According to Frederick Brooks, in his book The Mythical Man-Month, adding more workers to a project which is already behind schedule, is likely to make it even farther behind schedule as new members are brought on board.

Take the time to consider whether you can beat this effect before attempting to grow your group.