Adverbs tell is when, where, why, and how. Does the phrase including John do that or does it tell us more about the noun phrase Several people?
Ex: Several people, including John, went to the party.
Ex: Several people went to the party, including John.
Including in the second example, according to the Concise Oxford Companion to the English Language | Date: 1998 (cited here), functions as a preposition:
Participial prepositions and conjunctions Apparent exceptions to the rule that participles should be properly attached are a number of participle forms that now function as prepositions, such as following in ‘There was tremendous clearing up to do following the storm’, and including in ‘We all enjoyed ourselves, including the dog’; and participle forms that are now conjunctions, such as providing (that) and provided (that) in ‘Everything will be all right, providing/provided you don't panic’, and given in ‘Given the difficulties, I'd say it was a success.’