Wikipedia:Backdoor canvass

Mentioning other editors not currently involved in a dispute who have previously or separately disagreed with your opponent so that you can "courtesy notify" them is obvious canvassing, no matter how you try to spin it. Proper conduct dispute resolution is best done by providing diffs of their behavior, not by making a shopping list of folks who agree with your point of view.

Making your case
When "making a case" at either of the administrator noticeboards (WP:AN or WP:ANI), it is often tempting to highlight the fact that other editors have had similar disagreements with, or have been similarly aggrieved by, your opponent. There is logic to the argument that someone who is in conflict with many people might just be a disruptive editor (if that many people are telling you that you're wrong, perhaps you're wrong). This is reinforced by other consensus ideas that some editors simply refuse to listen to the community or sometimes just won't let something go.

Nonetheless, Wikipedia has very strict rules about canvassing and attempts to draw otherwise uninvolved editors into your dispute in order to boost your "case" will likely still be considered canvassing, regardless of how disruptive the other editor has been.

More harm than good
Administrator noticeboards are watched by many and frequented by many more again. Having neutral third parties comment on the case is often more valuable than a conga line of editors showing up to have their whack at the piñata.

And...
There is a good chance that if those other editors have a different but ongoing conflict with the editor in question, they will see your compulsory ANI-notice on the editor's talk page and will be prompted to offer their thoughts if they feel they have something to contribute.

Of course...
None of these ideas mitigate the need to notify someone that they are the subject of an administrator's noticeboard discussion. Courteous editors will often also notify those directly involved who aren't the subject in an effort to afford an appropriate right of reply.

There are other times where endorsement of particular cases is required or where specific groups (like ArbCom) will need to make contact with all those considered to be involved in a particular issue. This essay does not relate to those instances.