Attempting to create a script that will use ssh or another login method to allow many users to connect to 1 server

I currently have a script that connects to user@server.com via sftp. This is using a ssh key created on my current machine that I'm using for testing purposes. Below is a quick example of my script. I've tried finding a solution online but I don't seem to be looking in the right locations.

sftp user@server.com<< EOF

put file.csv

exit

EOF

What I want is for many clients (lets say a thousand to represent scalability) to be able to run this script and automatically connect to the server without each one having their own ssh key that we need to approve on the server side.

Edit: I should have clarified. That isn't my full script. I currently have a SSH key setup on my account that allows me to login to the server while testing my "working" script. The problem is that I need to be able to distribute this to many users, so I'm looking for alternate options besides making them each create a SSH key while also having to accept hundreds of public keys on the server side.