"Go" requires movement. We are in His service to come or go at His direction. This is a command concerning obedience, not a call to foreign missions. Obedience is the key to discipling with authority.
Sermons urging believers to leave their present place of service for a distant mission field use this text. Yet today America needs missionaries! The discipling begins where we live and continues to where the Holy Spirit leads, as with the Apostle Paul.
The Holy Spirit directed Paul in his missionary calling to go into Macedonia. Peter was led to preach to Cornelius. It is central to discipleship and evangelism that we be in tune with the leadings of the Holy Spirit. He leads us in His own sovereign way and according to His timing, not ours.
In my experience, “Upon being lead...” is the way all missionary calls have materialized. The individual may feel a compulsion to “go” as a missionary, but the candidate cannot go unless "being lead" by the Spirit. This is the Shepherd Principal found in John 10:3–5, 26, 27 and Acts 16:6–10.
The Christian life is Christ’s life in which we are privileged to participate. Therefore, “upon being lead...” reflects our participation with Him, under His Lordship.
Related reading: Making Disciples: What Christian Students Should Know; See What God Can Do!; The Great Commission