You can lead a person to iNaturalist...
...but you can't make them upload.
Person on a listserv (paraphrased): I saw this interesting thing that I'd like help identifying. Can anyone look at it for me and explain the behavior?
Me: I recommend submitting it to iNaturalist.org and adding it to any regionally or taxonomically relevant projects to help it get noticed. Email me your user ID and I'll try to help tag some people who might be helpful.
Person: Thanks for your suggestion. Would you be able to put me in direct contact with anyone who could help with the ID? It would be great if I could talk directly with a specialist, since I have a number of questions about the behavior I observed.
Me: I have one idea of an iNat user who does a lot of macro invertebrate monitoring who might know more. Hopefully on iNaturalist you'll come across a specialist and can ask. After all, it's a social network for naturalists. :-)
Person: Hello--were you able to find anyone who might be able to help me identify this species from the photo I sent?
Me: Did you upload it to iNaturalist yet?
Person: I appreciate the suggestion, but I'd much rather contact an entomologist who works with the group--I was hoping you might be able to recommend someone.
Me: headdesk
I should have just sent him to BugGuide.