其实这是一个倒装句,把句子要强调的serving this great variety of people放在了句首.而后面的enrolling more than 12 million students 则是一个后置分词短语,修饰前面的3400 colleges and universities.正常的顺序应当是:
About3400 colleges and universities have enrolled 12 million students to serve this great variety of people.
大约3400所大学院校组织了1200万名学生来为这些不同身份和层次的人群服务.