To answer this question, let us first crack the metaphor of this strange parable, that taken at a face value, approves cheating for one's personal benefit.
a) Within the context of the parable, the friends are those people whom the clever steward has helped by his unjust reducing of their debts to their - and his - master. They will definitely
become his friends exactly due to his compassionate cheating and alleviation of their condition. Thus, in earthly realm, when he will be left without salary, those families whom this steward benefited by his compassionate cheating, will take him in and provide him food and shelter, as friends obliged to.
b) spiritually the "master" is God, to be sure; the "debtors" - all humans; the "steward" - any of those debtors, any particular man. When does one act as a "clever steward" so as to deserve approval of "master", i.e. God? Definitely, when understanding his own sinfulness, i.e. indebtedness to God, a man alleviates this indebtedness to God through his being lenient and compassionate to sins and trespasses of others. Not judging others is a condition of not being judged by God (Luke 6:37). But what does it mean? If I am an alcoholic, which is a dangerous sin, and I am lenient towards other alcoholics, not judging them, does it mean that my sin is blotted out due to my compassion to others suffering the same plight? Not exactly, for only such a compassion does not heal my sinful inclination to the alcohol. My desire and resolve to get rid of this sinful inclination is sine qua non for it to be forgiven by God. But if I have this necessary condition, fight my sin and, moreover, have compassion and solidarity to others who suffer from the same sin, then and only then it means that I fight the sin in Christ's Spirit, humbly and with a thankful reliance on divine Grace, and no only my efforts, and that, moreover, I am ready to help the others through experience of my graceful fight with the sin. Thus, as I myself, through my humility, ask God to show me mercy/grace and not justice, for "if You will give retribution according to what our trespasses justly deserve, who will withstand it? But with You abides mercy" (Psalm 130:3-4); so, also, with respect of others, I "cheat" God's justice, like that clever steward from the parable, and treat other people mercifully, that is to say, just as I wish God to act with me. And such "cheating" is indeed pleasing God whose "Mercy triumphs over His Justice" (cf. James 2:13).
Now, if in the parable the friends are other people whom this steward showed compassion and reduced their debts, in spiritual understanding those friends can be again people, who you were compassionate with and treated with divine mercy; but since you are also a sinner, they will treat your sins and trespasses also similarly leniently and mercifully, and thus, divine Grace will start working towards you through other people, as their thankful answer to your lenient and merciful attitude towards them. For what is to receive somebody in eternal dwelling unless to be ready to forgive this somebody and not grudge for him divine forgiveness, which is the key to the eternal dwelling in the Kingdom of Heavens.
Alternatively or complementarily those friends can also be angels, for if one behaves like described above, then he likens himself to angels, and the latter will definitely become friends of somebody so like them, who treats the people under their protection just like them.