I think in order to hold a glass of water, the following muscles will act:
Arm: mild flexion by biceps brachii.
Forearm: mid-supination by biceps brachii and supinator & flexion of the fingers by muscles of the anterior comparment of the forearm; flexor carpi radialis, flexor carpi ulnaris, flexor digitorum superficialis, flexor digitorum profundus, flexor pollicis longus.
Thumb: opposition (flexion of the thumb at the first carpometacarpal joint) by opponens pollicis & flexion of the first metacarpophalangeal joint by flexor pollicis brevis.
Little finger: opposition by opponens digiti minimi & flexion by flexor digiti minimi.
This may not be totally accurate, but I guess it's pretty close.