How do I give a user sudo permissions?
Rather than give geoff sudo privileges, consider adding the account to the admin group so that it inherits the admin group sudoer privileges. This would be the more correct way to do things.

To add geoff to the admin group you'll need to run the following as the admin account:

sudo dseditgroup -o edit -a geoff -t user admin


You may also want to consider putting geoff in the wheel group too:

sudo dseditgroup -o edit -a geoff -t user wheel


The wheel group is a BSD-ism, where OS X has its roots. In traditional BSD systems the wheel group was used to keep a collection of users who were allowed to become superusers using the su command. It's not strictly necessary to be in both admin and wheel but anyone setup as as "Administrator" on the machine through the UI for adding a user is in both so it can't hurt to replicate that setup.