This guide will show you how to do it:
1. If the person you want to make an admin is not yet a fan of your page, ask him to push the Like button and become one of them. You need this in order to give him the admin status.
2. On the left side of your Facebook page, right below the quantity of fans, click on the link that says like this.
3. Once you have done this, you will be looking at the complete list of fans of your page. On the right side of each row, you will see a button that says Make Admin. Look for the row of the person you want to make an admin and click on its corresponding button. Facebook will ask you to confirm this action. Since you are the creator of the page, you will be able to remove this new admin later, any time you want to.
To add many people, just repeat this procedure with each one of them. This way you will no longer be a lonely administrator.