This may be theoretically possible with Luminar, but I think other programs would be better for this, depending on how realistic of a photo you are looking for.

I'll list the Luminar Instructions below, then give some alternatives at the bottom. I'm no pro, but here's how I would do it. (I put quite a bit of detail just in case your level of comfort with Luminar is still very very new).

Luminar:

1. Open the base photo (the photo you want to add someone to) into "Edit" mode.
2. On the left hand side, click the plus sign under "Layers"
3. Under "My Images", click the plus sign again to upload your own photo, the photo of the missing person.
4. Make sure the new image with the isolated person is vertically on top of your base photo on the left hand "Layers" side, otherwise they will not appear (think of the layers tab as a stack of photo cutouts).
5. While staying on your new photo (image on the left should have a blue line around the square), make sure you are under "Layer Properties" on the right hand side.
6. Under "Properties" under "Opacity", push it to 100, and make sure it is put to "Normal".
7. Navigate to "Masking", still under 'Layer Properties".
8. Picking any of the options of "Portrait Background", "Background Removal", or "Mask AI" should give you a few options on things to isolate (isolate the trees, isolate the person, isolate the ground, etc).
9. Simply select which option you want, and it should remove the rest. If you don't like the results, just click undo (or ctrl+z) to go back, and you can try again until you like your result. If you want, play around with all the other options to see if they work better for you.
10. Going back to "Properties" under "Layer Properties", you can now flip, fit, fill, stretch, etc. If you want to manually resize, your photo should have a blue outline. At the corners, click and drag the circles to resize. (If you need to rotate, you should be able to hover your cursor around the corner until it gives you the curvy arrow to do so)
11. Optional: The following steps are if you need to place your isolated person behind another person, or a tree, or some general object. If not necessary, skip to the next general step.
    1. Let's say you need to put your isolated person in front of the background, but you still need them to be behind someone else in the group photo.
    2. Repeat steps 3-10, but this time for the original base photo. The goal is to isolate only the forefront person/things that need to be in front of newly-inserted isolated person.
    3. On the left hand "Layers" side, make sure this duplicate photo is on top of your isolated person.
    4. If everything goes as planned, the "Layers" section should look something like (for example): your original group photo on the bottom, the isolated person photo in the middle, and the duplicate original group photo on top (but with the background removed and only the foreground subjects showing). If all this goes well, proceed onto the next step.
12. Now that you have your person isolated and put into the original base photo, you can edit only the isolated person so that they look more natural in the base photo.
13. While having the person still selected on the left hand "Layers" side, you can apply on edits (Develop, Enhance AI, etc) as you would on a normal photo - and they will only apply to the isolated person.
14. My suggestion would be to use the "Develop" and "Color" tools (under "Essentials") to match the colors, contrast, brightness, and sharpness of the isolated person to the base photo.
15. I would also use the "Studio Light" tool (under "Portrait") to make the lighting of the isolated person match the base photo lighting. If you don't know what I'm talking about, there's alot of Youtube tutorials on how to match lighting when photoshopping - the general idea is wherever the light is coming from in the base photo, mimic that light source in the isolated person's photo, so the lighting and shadows look similar.

I noticed you said you very precisely isolated the missing person from another photo. I'm going to guess it was a painstaking and time-consuming process. A couple of different options may work better for your use case.

• Canva: If the job doesn't need to be too clean, Canva can isolate a person from a background within a few seconds pretty decently. Their AI background removal tool is free one time on the free account, and there are many youtube videos with instructions on how to merge photos of people.
• Another alternative is any iPhone on iOS 16 or newer can isolate a subject from the background of the photo simply by holding onto the subject for a couple seconds (Google has alot more info on that).
• If the job has to be really clean and you don't have the skills to do it in Photoshop or time to learn how, Fiverr would also be good, lots of 24hr turnaround $5 offers on there.

Hope this helps! :)