It depends upon what level of access you wish to give them.
If you want to give them all access to your GoDaddy account, except prevent them from making purchases or seeing your billing information, just click on the Account Settings menu after you've logged in, and select "Delegate Access." On the next page, click on "Grant Access," fill out the fields, and select the "Manage Products" option. This will allow them to make changes to DNS, your domain, and other related settings. It will not give them access to CPanel.
I don't believe there is a way to create another CPanel user, so you'll just have to share that login information with them. You may want to change the CPanel password to something different than your GoDaddy password before you give them that information.
If you just want to allow them to make site uptates, you can set up an FTP account for them. If your site is in Wordpress, you can create a new Wordpress user and give them the appropriate permissions.
Hopefully that will give you some help!