The first conference met at Basel in 1925, on the four hundredth anniversary of the first adult baptisms at Zurich in 1525, which Anabaptists count as the beginning of the movement. It was a commemoration before it was an institution, and the institutional form arrived slowly: assemblies at intervals, a general council of delegates from member churches, an executive committee, and a small staff.
What it is not is a synod. Member conferences keep their own polity, their own confessions and their own discipline, and the Conference has no power to legislate for any of them, to ordain, to discipline a member church or to speak in its name without leave. It describes itself as a communion of churches, and the practical content of that is mutual recognition, shared study and the ability to act together where members agree to.
The nearest thing to a common statement is the Shared Convictions, adopted in 2006: seven short paragraphs on God, scripture, discipleship, the church as a community, peace and nonresistance, and worship, framed as what the member churches hold in common rather than as a creed anyone must subscribe. The wording was deliberately kept brief. A confession long enough to be precise would have been long enough for member churches to disagree with, and the body exists to hold them together rather than to sort them out.