The root yuj (i.e. “yoke”) is very fertile in Pali, as befits a culture where the chariot and plowing are so prominent. One fairly rare form it takes is the term sampayojeti. Here, if the prefixes may be read literally, it reads as a double-emphatic, “tightly bound up with”. The PTS dictionary, following the commentary, gives it two meanings: to associate with, and to quarrel. It does seem a little odd for it to take on such opposing meanings, so let’s see if this is really justified.
We find a very clear meaning in MN 93, where it refers to the “mating” of a mare and a donkey.
In Kd 20 it refers to some bad monks who have intimate relations with some nuns. Presumably it’s not sex, as then they would all be expelled, but is probably a more generic term. It could be physical, or simply “overly close association”.
A similar sense is said regarding monks getting too familiar with laypeople at Pvr 17.
In Kd 11 there’s a list of things that a monk who is undergoing probation for various disciplinary offences should adhere to. This includes sampayojeti with the monks. Here, the commentary says it means “argue”. However, the other items in this list are all things that a monk may normally do, but are prohibited while under probation. I think it makes better sense to read this is a similar sense, as saying that a monk under probation should not get overly familiar with his fellow monks.
At SN 11.24 we have the story of two monks who were sampayojeti, but one of them transgressed against the other, and they had a falling out. Again, the commentary says “argue”, but this seems unnecessary. In fact, the verses at the end speak of not losing ones’s friends, so it seems that here, too, the sense should be read as “in a close relation with”. The fact that the relation led to such a falling out suggests that it was not entirely healthy.
Finally, MN 77 speaks of monks who, having been sampayojeti with fellow monks, disrobe. Here, too, it seems unnecessary to read as “having argued with the other monks”, but instead “even though they were close friends with the other monks”.
The striking thing is that, in each case, the sampayoga heralds some kind of unfortunate outcome. This is in stark contrast with the very positive portrayal of “friendship” (mittatā) in the EBTs.
It thus seems that we should reject both the neutral sense of “association” and the sense of “argues” entirely. It means “overly attached”, “having overly familiar relations”, “unhealthily close to”.