Just use the normal propaganda stuff.
First you need to estimate what the potential new iNatters are interested in.
Some like the nice flowers like roses, some more like animals like flipper the dolphin or black beauty the horse or lassie the cute dog, some may more like fish to eat or mushrooms in a crime scene or just some nice singing birds.
Then you make them nosey about some species in the field of theyr interest, some thing they dont know, a new bird in the region, or the plant of which they dont know the name but which is useful for some thing, may healthy or food or tea.
Then tell them about inaturalis, this super great app which can ID all life forms.
Explain them how they can use the app to ID some observations, tell them how to do good observations, like to picture mushrooms also from the bottom side, or also the stem from plants or the open wings from butterflies (what ever they are interested in, just tell them how to do good observations).
Then tell them how to use inaturalist for theyr interest.
Do not just tell them to use inaturalist and then let await any thing.
Give them some perspective and also do this with some enthusiasm.
Its totally amazing, plants are so interesting you can find your own tea in the woods, you dont believe that because you dont know the plants, but if you use the inat app, you can picture the plants you see and soon you get an ID, then you have the name and you can look it up on wiki and then you will see there are many useful plants, you will learn so much it is totally amazing, and you even will help scientists, your pictures can even get some scientific status, you dont believe how amazing this app is and how much you will learn i a short time.
So it is not just about picturing life, but there is also a perspective to why a non scientist should use inat, because they get the names and they can look it up on wiki and then they can use it in theyr kitchen. Thats much more than just to picture life because it makes some professor happy.
Or the animal lover story…
Its so amazing what you learn about animals, first you think its just a slimy snail, but a snail is not a snail, no its a Arion or a Limax, and if the Arion eats your salad you need more Limaxes which eat up the eggs of other snails, and if you dont know the difference between arion and limax, just ask inat, inat is amazing they know every thing about wild life and you will learn even more about animals or fungus on your plants, on inat they can identify animals even by traces, every one should try this, once you get into it you will never stop inatting.
Just make up some catchy story for the potential new inater individials.
Add the common stuff as you know it from the internet or newspapers.
”its amazing”, “it can … any thing”, “everyone”- or “most”- or at least “many”-”use it”, “you will want it”, “why you guys dont know it? (whats wrong with you!)”, “even such (what ever) people use it”, “every day they get more and more inaters, its seems to be very trendy (may with a odd face)”, …
With scholars or may young students you may also can apply the reverse psychology trick.
”Find 10 odd wild life-forms in nature, picture them and identify them your self. (you should try to identify them your self, but we will not check if if you have used inaturalist)”.
This gives the impression that they have to do some thing, what is boring or they dont want or do not have time for, and it will suggest them that the short “forbidden” way ist inaturalist, but you already let them the door open as you say that you will not check it. So they get the impression that inat is a secret cheat app.
Once they are in inat and they have found theyr interesst, they will keep using it or at least they will come back by the next chance, or may just even remember it as the app they used to cheat some homework and then they may will recommend the app to other “cheaters”.
Also if you expect them o appear on inat, keep an eye on inat and the specific region of the new inaters, if no one else does, try to ID theyr observations to give them some fast success, use the comment section to tell them how to make better observations or how to ID animals them self, explain them odd stuff about the difference of some similar bugs, or telll them a odd story about a mushroom that looks similar to an other mushroom. Keep them interested and even open up some new perspectives or interests up to them.
Make inat theyr new drug :))
You can also use the Pokemon2Go-Effect https://wild-achievements.mywild.co.za/
”It is like Pokemon-to-go, but it makes sense, because you learn a lot and help science, if you beat me you are better than me.. try it your self, i bet you beat joe on day one, but if you beat Josh you are my new king”
Here you can also compare user versus user or just watch your self https://elias.pschernig.com/wildflower/leastobserved “if you beat this guy on limax you are the king, but to beat him on sunflower will be hard”.
A good one is also to find endangered species “If you find a lot of them in a region they may must protect the region and call it a natural reservate… be the grain of sand in the system, get an active activist”. (a good one for anti-authoritarians)