Just like to check something here...
We know that additive phrases such as 'in addition to', 'also' etc do not alter the subject from singular to plural like the word 'and' does.
So: additive phrase + singular = singular
So does it also means an additive phrase preceding a subject will result in a plural subject. For instance: The boys, in addition to the girls, are participating in the games.
So does additive phrase + plural subject = plural ?
Yes, "The boys, in addition to the girls, are participating in the games." This is correct.
Even this - The boys, along with Polly, have arrived. boys - plural, have - plural.
Along with Polly, the boys have arrived. - better construction. boys - plural, have - plural.