How can I detect if a player is inside a certain area?

For this kind of problem I recommend using Region3

You construct it with Region3.new(), and give a Vector3 min and max as the constructor parameters.

From there, you can use workspace:FindPartsInRegion3(), which takes in a region, an instance to ignore, and an int value of max parts (sort of similar to rays).

Check them out here

Also, the FindPartsInRegion3 method

Edit to answer your question:

Create a new table to store found players.

Iterate over the table that gets returned when calling the FindPartsInRegion3 method. For each part, check if its parent contains a humanoid; this means it is a player (likely).

From there, you can use the GetPlayerFromCharacter method from the player service (game:GetService("Players"):GetPlayerFromCharacter(char)) which takes a character as a parameter and returns the player - the character will be the parent of the part you're currently iterating over.

You can then check the table to see if the player already exists there; if it does, ignore the current part. Otherwise, add the new player to the table.