That website isn't very trusteable (the information), it doesn't give totally true info. For example, it says that my router's default gateway address is 192.168.2.1 but in the picture it shows 192.168.1.1 (it's 192.168.2.1) and it says the default password is blank, which is wrong.
The first argument in the function should be a string but you didn't use the quotation marks (you wrote Vehicle locked. but it should be "Vehicle locked.").
I would not use tables for such data, and the code should work (perhaps you're using ACL groups but you have another kind of administration system?). I also recommend you doing an assignment instead of using the table.insert function, it's faster:
if ( garage and nmieter ) then
mieter[ garage ] = tostring ( nmieter );
end
I think the first would be the fastest. If you do it the second way, you have to keep updating the table.
Also, if you want to check lots of players, I would recommend just getting a list of players eligible for the thing you want (if applicable).