What is the grammatical function of 'where to meet them' in this sentence?
This is an infinitival interrogative clause with interrogative word where fronted - compare with declarative infinitival:
to meet them where [declarative]
where to meet them [interrogative]
These usually have the force of directives - where we should meet them (CaGEL p985)
Where should we meet them? [interrogative]
Where are we to meet them? [declarative infinitival]
Did they tell us where to meet them? [interrogative infinitival]
As far as clause structure is concerned, the function of where to meet them is that of complement to the verb tell.