...credit go up?
my credit is low but all my cards are paid off. my fiances credit is great and we are trying to fix mine

No. It only depends on the balance on your own cards.

your good credit will go down & it will hurt you if you add someone w/ bad credit. why would you do that???!!!

Both previous posters are simply wrong.

The person with bad credit will get the payment history of the person with good credit on their credit report. This will increase their score and it will not hurt the person with good credit at all.

Just be sure that you are added either as a joint or co-signer and not a authorized user because the way FICO credit scores are calculated is going to change next month and authorized users will no longer get additional points.

It all depends on why yours is low? Need more information.