1 Corithians 6:15-17
Know you not that your bodies are the members of Christ? Shall I then take the members of Christ, and make them the members of an harlot? God forbid. Or know you not, that he who is joined [κολλωμενος] to a harlot, is made one body? For they shall be, as he saith: the two shall be one flesh. But he who is joined [κολλωμενος] to the Lord, is one spirit.
The central argument is that in the conjugal act, the two become one flesh. Similarly, when we become members of Jesus Christ, our spirit and God's become "one spirit." That is, just as two fleshes united become one flesh. Therefore, he says, flee from fornication as joining the body of Christ, and God's Spirit, to a harlot. To commit fornication by joining yourself to a harlot, you have committed, as it were, adultery against Christ.
The Greek Word
That is, the 'joined-one [to]' x. This is what the verb means in Greek and in context, in both places. In my understanding.
Specifically, the word is made up of a root verb κολλάω (kollao) (verb meaning to join or cleave) and an ending μενος (menos) which stands grammatically for '< one described accurately by this verb'. That is, "who is joined [to]" describes this hypothetical person; the man joined to a harlot in one flesh, and the man joined to God, as it were, in the one spirit. Without saying anything of the time period during which it took place, or its remaining true to the present. Only that it is true of said hypothetical person that at a time he was accurately described this way and/or still is.
For example, Sirach 18:17:
οὐκ ἰδοὺ λόγος ὑπὲρ δόμα ἀγαθόν καὶ ἀμφότερα παρὰ ἀνδρὶ κεχαριτωμένῳ
Behold, does not a word surpass a good gift? And both are with a man graced.
Now in this case the κε means 'has already been [graced] and still is.' But the ending is the same in meaning (even though it is in the dative form [μένῳ]). And that is: someone now defined by the fact that verb x is (or was) true of him. He has been graced. The Christian has joined himself to God.
Another way of translating 'μενος' verbs is, and to be archaic, 'a man' followed by the relevent rendering of the verb in English: "a man [having been] graced."
κολλωμενος simply means 'the one joined [to]'. And this is synonymous with 'he that joins or has joined himself' (active).
In my understanding, the context is how we determine whether it is passive or active. In the context of the passage in 1 Corinthians, it is hypothetical, and so is agnostic to the passiveness or activeness of the joining: it just means 'the one joined' or 'the one who joins himself.'
Parenthetically, the Latin Vulgate has:
Qui .. adhǽret Dómino
That is, the one adhering to the Lord. Again, quite neutral.
I am open to correction by those who understand Greek much better than I.