To obtain a list of the cards with a particular group number you could use a query something like
c:extract value where value..widgets.group.text=2 from deck.cards
(Note that if a 'group' field doesn't exist it will behave the same as a card in group "0"; you probably want to count from 1 for your groups)
Given that list, you can pick a random item and navigate to some card like so:
go[random[c]]
Or, all at once,
go[random[extract value where value..widgets.group.text=2 from deck.cards]]
Does that make sense?