Small human complex systems, here called co-present groups, are found across all fields of human social life. Complexity thinking suggests why this is so: that these groups, irrespective of formal content, have a meta-function of providing maximum complexity to manage the "indeterminacy" or "uncertainty" that characterises the most complex of human social issues. This claim depends on an understanding of the functioning of these groups as being characterised by irreducibly complex intersubjective (person to person) relations, which are involved in the processing of raw human experience ('affective processing'), to produce shared meaningfulness. An example of the use of co-present groups in medical continuing professional development (CPD) is given.