There is well-known research that shows people’s efforts quickly diminish as team size increases. This is because team members reduce their input when they feel less responsible for the output. Use the smallest team possible to achieve your goals.
This may mean including slightly fewer members than the task at hand requires. If reducing the size of your current team is not an option, consider dividing up a complex task into manageable bits, where every member of the team is accountable for one part of it. Or you can create a sense of urgency about the task so everyone’s invested. If you see people not doing their part despite your efforts, be transparent about everyone’s performance so slackers can’t hide.
Adapted from “Why Less Is More in Teams” by Mark de Rond.