The act of becoming a formal student of a fully transmitted Zen teacher is called shoken. When and with whom to do this is a very personal decision. Shoken is not a requirement for participation in any aspect of community life.
Each of the transmitted teachers expresses the unity of harmony and difference in their style and emphasis. All are teaching the same essential liberation through the same means, yet each is doing it with their own emphasis, style, and approach. At a certain point in your spiritual training, it will become valuable for you to clarify which approach resonates most deeply with what you find personally liberating in the Dharma.
If you wish to make a formal spiritual commitment to Zen practice, and feel particular affinity for one of the transmitted teachers, you may wish to talk with that teacher in dokusan about shoken. Becoming a formal student clarifies the teaching relationship in both directions, in a variety of subtle ways. It is traditional to offer some small token gift to the teacher at the time of making your formal shoken request. You will also be encouraged to write a statement of spiritual autobiography, which can range in form from a haiku, to an essay, to a more massive work—as suits you. The third element of the Shoken ritual is to discuss with the teacher your plan for exploring the Dharma for the next year.
Before and after shoken, you are always welcome and indeed encouraged to go for dokusan with any of our teachers.
The few practical implications of shoken are these: if you at any time wish to ordain as a Zen priest, it will generally be your shoken teacher who ordains you; and in the event you follow the Zen path long enough to become a lineage holder yourself, your lineage will flow through your shoken teacher.
If at any time you would like to change who your shoken teacher is, you are always free to do that (just talk with with your current shoken teacher first).