I did it in Lion. You need to be a member of the wheel group to use sudo as I understand it, an administrator account should have this access.
If you're still stuck you can attempt this solution I found online which a few people said works in Lion:
If you're still stuck you can attempt this solution I found online which a few people said works in Lion:
Reboot into single user mode (hold Option S while booting until it switches to text mode)
At the single user prompt, type: /sbin/fsck -fy
This checks the filesystem integrity
Then type: /sbin/mount -wu /
This mounts the root filesystem as read/writable.
Next type: /bin/chmod 1775 /
This will correct the permissions for your / directory
Type: /bin/sync
This will make sure the change is written to the filesystem
Type: exit
Your mac will continue booting as normal and life should be good! Sudo should work.