这个函数是对 linux C函数 initgroups() 的包装 node.js 官方文档很含糊，还是看 linux C函数文档的解释！很清楚明白。
The initgroups() function uses the getgrouplist(3) function to calculate
the supplementary group IDs for the user specified in name. This group
list then set up for the current process using setgroups(2). The basegid
is automatically included in the group list. Typically this value is
given as the group number from the password file.
参考: http://www.man7.org/linux/man-pages/man3/initgroups.3.html理解 process.initgroups(user, extra_group),布布扣,bubuko.com
理解 process.initgroups(user, extra_group)
原文：http://blog.csdn.net/liveinjs/article/details/36935501