Problem description:

runtime error: type mismatch
proc name: Login (/mob/Login)
usr: KirbyAllStar (/mob)
src: KirbyAllStar (/mob)
call stack:
KirbyAllStar (/mob): Login()

what does this mean and how do I fix it?

It means you're trying to add a string to a number.
var/n = 5

n += 1 // Valid
n += "blah" // Invalid
In response to Nadrew
Ok Thanks i had This
player += src.key

When I should have Had this.
player += 1
In response to KirbyAllStar
Actually you were right the first time, but player needs to be an initialized list. Better to keep a list of players than a count of them.

Lummox JR