There are a few people to whom the dictionary is useful.或者There are a few people the dictionary is useful to.
to whom 在定语从句中是状语成分.
一般情况下类似于to这样的介词可以放在定语从句中,也可以放在定语从句句首,就像这句话.但是主要有两种特殊情况：一是,当先行词（就是which,who,whom这种引导定语从句的词）是that的时候,介词一定不能放句首,一定要放在定语从句里面；二是,当某些介词是和动词连用的,是与动词连在一起的动词短语的时候,介词不能与动词分开,也就是不能放到句首,比如look after(例句：The patient whom the nurses have to look after is not well.就不能写成The patient after whom the nurses have to look is not well.)