When using Darkrai & Cresselia LEGEND's "Moon's Invite" attack, can you put more damage counters on a Pokemon than it has HP left?
Yes, you can put as many on a Pokemon as you want to, as long as you have enough to move, of course.
Source: Triumphant FAQ (2010-11-04), PUI Rules Team (2010-11-04)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_100
HGSS4_99