Enquire and inquire are often just different spellings of the same word. Where the two are used for the same purposes, inquire is the more common form. This extends to derivative words (inquiry, inquirer, etc.), and it is the case throughout the English speaking-world.
There is one qualification to this. Some Britons make the distinction that enquire and its derivatives apply to informal queries, and inquire and its derivatives to formal investigations. While this distinction appears widely borne out in more carefully written British texts, it is less pronounced in more informal types of writing (some news websites, some blogs, web comments).
At least one job-seeker called Bannon the next morning to politely inquire if he had, indeed, taken the resumes with him. [The Connecticut Mirror]
Gratl told the inquiry earlier that the federal government’s application is too broad. [Vancouver Sun]
A criminal inquiry into phone-hacking at the News of the World could be reopened and staff prosecuted following legal action by some of its alleged victims. [Guardian]
Talks not serious, but Rockies inquire about Rangers all-star Young [Denver Post]
One of the vintner’s shareholders sought to inquire about a wallaby pictured among the vine. [Sydney Morning Herald]