So I am BRAND NEW to textadventures, however, I am goal oriented.

Is there a way to keep track of individual players (say by putting a password in the game for each one? I can do this manually, actually.)

I'd like to give a small number of people ONE chance to run through the game (as part of a bigger game.)

I think I understand what you are asking.
I'm not sure how the squiffy site handles this. Usually if you leave, when you come back you'll see your last page but you can't click on anything.
I'd think one way that might work is to post a code when players hit "save points". Then you could enter this code on later plays and go directly to that point. However, saying this, it would be difficult to maintain inventories and status.
To that point, maybe you could develop a code that would include all that. Say, the 100,000s would be a thousand possible codes, and the numbers below that would control inventory.
So, let's say the save point is 102,500.
A candle is worth 10.
A shield is worth 15.
A bandaid with worth 1.
So at the save point, you'd tell the player to remember the number 1025026 (the addition of save point and the sum of inventory points.

I think if you want to make sure that people only play once, you'd need something on the server side to keep track of them.

If you've not got a server you can use, you could bodge this with a site like file.io - a file sharing site that allows you to share files that are automatically deleted once downloaded. You could host one of the game's images or something on file.io, and have javascript to display a "Sorry, you've already played this" message if the image comes back not found. So the "password" would be the set of random numbers that are used to identify a given file.

If you wanted to allow "saving" the game, you'd have a small piece of script which uploads a simple text file containing the game's current state, and gives the player their new "password". So it's basically a save/load system, where every save file has a unique "password", and is automatically deleted immediately after loading. You'd manually create a save file for each of your players at the start position..