The [source:trunk/tests/testapp/models/m2m_intermediary.py Many-to-Many Intermediary] test case already shows how this is structured.  What I add here is a set of parameters for the fields in the intermediary model that allow smooth inline editing in admin.

{{{
from django.db import models

class Individual (models.Model):
    class Admin: pass

class Event (models.Model):
    class Admin: pass

class Participation (models.Model):
    event = models.ForeignKey(Event, related_name='participants', edit_inline=True, blank=True, default=None)
    individual = models.ForeignKey(Individual, related_name='events', edit_inline=True, blank=True, default=None)
    role = models.PositiveSmallIntegerField(core=True, blank=True, default=None)
    class Admin: pass
}}}

 1. {{{edit_inline=True}}}
 1. {{{core=False}}} ''# So that when an {{{Individual}}} is being created along with a few {{{Participations}}} created inline, the {{{Participation}}} records will be valid even though the {{{Individual}}} doesn't fully exist yet.''
 1. {{{blank=True, default=None}}} ''# So that untouched inline records will have blank core values and thus will not be created.''