Phil 1:6 is the key verse.
Thank you letters are hard to write. Philippians is such a letter. It is the most personal letter of Paul addressed to churches. That church had sent Epaphroditus and money to Paul. The emphasis of this letter is partnership.
We are partners with our mission partners.
The basis
We share together in God's grace.
Called by God, slaves of Christ Jesus
Past, present, even in suffering.
The future - God's grace will go on to the end, until the Day of Jesus Christ. All the yous are plural.
The expression
In attitudes - deep love and affection - do we have that kind of love? They entered into Paul's sufferings.
In actions - they sent Epaphroditus and gifts. Should we contact our mission partners? Paul prays for his readers' growth. True love is discerning. We need our priorities right. We need to pray that we will not be stumbling blocks.
The results
Thanksgiving to God
Joy
Love

Partnership only works when all are involved.