EDIT EDIT: Let me just confirm, your question is basically that nowadays we have a formal method of bay'ah to a murshid and a pir/mureed relationship. Ijaza of tasawwuf is passed down from peer to mureed in the term of khilafa. But how was tasawwuf passed down in the time of the Sahaba, tabi'een, and shortly thereafter, before the silsilas were formalised? What was the method for presenting ijazah before the system of bay'ah and formalisation/naming of tareeqahs?