tonight this cat was crying outside my back porch door. i went outside to see what was wrong and it could hardly walk and was SO SKINNY!!!!

here is a pic showing it’s body underneath the fur.

i put some food and water out.. and it started eating right away. but i can’t let it inside because i have a cat of my own. what should i do? leave it out to die?! it’s already so close to dying i can tell. but i NEED to keep it alive i have to save it. any advice??