Can I use Slowpoke's "Rambunctious Party" attack if my Bench is full?
Yes you can perform the attack, but all you get to do is look at the top 5 cards and then shuffle them all back into your deck.
Source: Undaunted FAQ (2010-09-09), PUI Rules Team (2010-09-09)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS3_66