I think the question is whether or not you can reasonably expect someone to learn harmony based on some attempts you might make with them in a rehearsal setting. The answer in my experience, after having tried on many occasions, is no. You can't.I get the frustration with some band member who you may not have personally been able to get to sing harmony parts at rehearsal. Don't over-generalize from that experience.
If someone wants to build a skill, there are ways to do it. But, as pointed out upstream, that's a different level of effort that someone needs to take on for their own reasons, not because someone in your band would like you to add something. It's their work to do, much in the same way as learning to read notation is their work to do.
If you want to take someone under your wing and mentor them on skill-building, that is a different thing that's going to require sitting down with them outside of the rehearsal setting and doing specific work, and it's going to take time.