How far should ISKCON devotees conduct marriages for uninitiated persons, supporters and congregational members?

ISKCON devotees should not be actively involved at all. Srila Prabhupada was very clear about that. Marriages can be a profitable treat for the temple or individual devotees being the priest in such ceremonies, but it shouldn’t be done under any circumstances.

Here is what Srila Prabhupada says about this subject:

“Regarding Volcer marrying, for the time being they can be married by the civil court without delay. Both the husband and wife may be allowed to associate with you and after few days, if you recommend for initiation, then you can send their beads and they will be initiated by post. At that time you can perform our regular wedding ceremony as usual. That will be nice. Uninitiated couples cannot be married by us. We shall not take the responsibility of an ordinary marriage maker. Our practice is to help devotees for advancing in Krishna Consciousness. In such activities, when there is necessity, we get them married also."

Letter to Jay Gopal, Los Angeles, 11th of January 1970