How many projects have you joined? Do you support? That can ripple back if you have common interests.
Write journal posts. (Not too many, not too often, with interesting content to pull in new members)
Invite people who you know, or hope, will be interested. Observers or identifiers ‘there’. Leave a helpful comment on interesting obs - have added this to … project, please join.
Identify on iNat, a lot. Leave comments on observations suggesting people join. See if there is a natural land group for the regions you name and reach out to them, if they have a social media person, they may help you promote it.
Leave a comment to say their observation has been added to the project or suggest that they add it to the project.
I have a project on culturally modified trees in Australia. Not something that is based on certain species. But certain species are more likely to include CMTs eg red river gums. I have found a few by googling where scar trees may be found and then looking at observations for the relevant trees in the same or similar areas. Leaving a comment asking if a certain tree is a known scar tree can pique people’s interest and they may go looking for others that might fit the bill. Posting journal posts on relevant material that can help people be aware of things in their area or what to look for are useful. But I think it is definitely a case of playing the long game.