You can withdrawl your agreement by calling the bondsman and telling them that you no longer want to be on the bond as you feel the ``criminal'' will not show up to court. You can sign off of the bail but they may arrest your friend.
Yes! In California, bonds can be posted twenty-four hours a day, seven days per week. But whether you can bail someone out of jail at any time depends on their situation. When someone is arrested, they have a first appearance soon after.