You can create the contact with the method - people.createContact, as you are probably already doing.
All you have to change is to specify a membership:
"memberships": [
{
"contactGroupMembership": {
"contactGroupResourceName": "contactGroups/PASTE_HERE_THE_NAME_OF_THE_GROUP"
}
}
]
Have a look at this sample.
Thx. As I suspected, there are no longer any independent groups in Google, only identifiers (labels, pointers). If I create a contact under a label and then remove this label from the contact, the contact still exists (in myContacts). A question about this, what about the groups Famlily, Coworkers, Starred etc.?
The groups Family, Coworkers and Starred are system groups and cannot deleted. When it comes to other groups, if you want to delete them - you can chose either the contacts should be deleted or not. If you want to remove a label/group from a contact - the contact will not be deleted. For documentation purposes if you can, please accept the answer (✓) that's been helpful to you - it helps other people that have the same issue in the future find the solution too :)
I did not want to know whether the system groups can be deleted, but what they are there for. They don't appear anywhere.
This question deviates a bit from your original question, but here you can find information what the purpose of system groups is.