1. As I reported to you in the other thread, many people consider its "function" to be a parenthetical element.
2. I have another explanation that may interest you. Consider this sentence:
...have made it clear that science, as well as political and social theory, is a battleground in the cold war between the West and the East.
3. As I said, many people would classify "as well as political and social theory" as a parenthetical element. That would be its "function."
4. But some people have another theory. Perhaps a theory that may please you more.
5. One scholar says that the subject "science" is modified by the prepositional phrase beginning with "as well as."
a. In other words, you can find people who use the word "preposition" to describe words such as:
in addition to
as well as
6. If you like this theory, then you can say with confidence that the "function" of "as well as his friends" is:
prepositional phrase that modifies the subject "Paul."
My source is a book named Understanding Grammar (1954) by Dr. Paul Roberts, page 278.