Calvin says in his commentary on Philippians 1:28.
but Paul in another instance, too, speaks of them as a manifest token or proof, (2 Thessalonians 1:5,) and instead of ἔνδειξιν, which we have here, he in that passage makes use of the term ἔνδειγμα This, therefore, is a choice consolation, that when we are assailed and harassed by our enemies, we have an evidence of our salvation. For persecutions are in a manner seals of adoption to the children of God, if they endure them with fortitude and patience: the wicked give a token of their condemnation, because they stumble against a stone by which they shall be bruised to pieces ... He proves it from this, that the endurance of the cross is the gift of God. Now it is certain, that all the gifts of God are salutary to us. To you, says he, it is given, not only to believe in Christ, but also to suffer for him. Hence even the sufferings themselves are evidences of the grace of God; and, since it is so, you have from this source a token of salvation.
So it would seen that the 'you' are those enduring sufferings on behalf of Christ. It does not seem to necessarily include all Philippians or all the hearers of the letter. Calvin continues that those sufferings and the ability to endure them are a gift from God and evidence of salvation. Since he mentions that ability to endure, it seems to imply that not all of the hearers or even all of those who are currently suffering have evidence of salvation. Only those who are graced to endure to the end may have reassurance of their election.
From what Calvin says, we can not know who is 'you' until later.
For persecutions are in a manner seals of adoption to the children of God, if they endure them with fortitude and patience.
For the hearers, all the hearers, it is an exhortation to endure. However, knowing who has been graced to endure is not possible during the suffering since it is the endurance to the end that is evidence of grace/election.
Whether or not evanescent grace may appear here does not seem to be addressed.