The verb meaning to sign up or to register is spelled enroll in the U.S. Enrol, with one l, is the preferred spelling outside North America. The more American spelling is now preferred in Canadian news publications, but enrol was traditionally more common and still appears in many contexts.
The spelling difference extends to enrollment (American English) and enrolment (outside the U.S.), but it doesn’t extend to enrolled and enrolling, which everywhere are spelled with two l‘s.
The word has taken several forms since coming to English from French in the 14th century. It was originally enrolly (an anglicization of the French enroller), but it was also spelled enroul, enrolle, and inroll.1 Enroll was common from the 17th century on, predating enrol by at least a century. English speakers outside North America took up the newer enrol around 1800, while Americans stuck with the older spelling.
These are U.S. publications:
In place since 2005, the GWU policy aims to provide financial certainty for families after students enroll. [Washington Post]
Just as high school enrollment increased during the 1920s, so too did enrollment at American colleges and universities. [The 1920's]
Every March, September and November, kids and adults who enroll in the eight-week Learn to Skate beginner session receive a free pair of ice skates. [Newsday]
And these examples are from outside the U.S.:
Procrastination, historically blamed for failure to enrol, is now the default means by which workers are funnelled into 401(k)s. [Financial Times]
Although enrolment and voting is compulsory, the penalties for non-compliance are not particularly onerous. [Australia: The State of Democracy]
Rivonia Primary School today won the right to determine the number of pupils it can enrol in a class. [Independent Online]