How does the Summoner fit into the group?

For a detailed look at each of these characters, simply follow the link provided below to access Gradesaver's summary of the prologue. In this summary, you will find the order (from highest to lowest) or status of each of the pilgrims. I assume that this is what your question refers to as far as "fitting in." Chaucer's pilgrims represent the different rungs of English society. The summary will supply you with this information in order of importance. 

The Summoner is red faced and pimpled, with narrow eyes. He has a skin disease across his black brows, and his beard (which has hair falling out of it) and he is extremely lecherous. There is, the narrator tells us, no ointment or cure, or help him to remove his pimples. He loves drinking wine which is as 'reed as blood', and eating leeks, onions and garlic. He knows how to trick someone.