A Group Resource is a binary file with a format described by the following C structure and constants:
#define GROUP_RESTYPE 'G'
#define GROUP_RESVERS '1'
typedef struct GroupResource {
// ----------------------------------------------------------
uint8_t resType; // (0x00) Resource type (Group = 'G')
uint8_t resVers; // (0x01) Resource version (Group = '1')
uint16_t maskBytes; // (0x02) Number of bytes in mask
uint8_t reserved1[12]; // (0x04) -
// ----------------------------------------------------------
uint8_t variableParams[]; // (0x10) Beginning of variable parameters
// uint8_t mask[*]; // (????) Bitmask (size is maskBytes)
// char name[*]; // (????) Group Name (c-string)
// ----------------------------------------------------------
} GroupResource;

