I don't see how this difference is relevant for exposing the functionality in python:

Linux:

int getgrouplist(const char *user, gid_t group,
                        gid_t *groups, int *ngroups);

MacOS:

int
     getgrouplist(const char *name, int basegid, int *groups, int *ngroups);

The only difference is that Linux uses gid_t and MacOS uses int.  The requested posix.getgrouplist() will take a python string and a python integer and return a list of integers.