The Spirit is responsible for implementing God's strategy. He is the tactician. He blends the body together. He knows the weaknesses and strengths of each local group. So, he knows each groups needs. So, he know what to add to complete each group.
He strengthens a group by adding different people. Because people have different gifts, he, in eternal wisdom, empowers different gifts in someone. He puts that someone into a particular family of God. That person lives out of his heart, expressing who God designed him to be and adds his unique contribution to the body, and fills an important need. That person matures. The body matures. The body bears fruit. And God is glorified.