some_list = [{'group_id': -1860624013}, {'group_id': -571707953}]
group_id_list = []
# просто цикл
for item in some_list:
group_id_list.append(item['group_id'])
for group_id in group_id_list:
...
# или можно использовать list comprehension
group_id_list = [item['group_id'] for item in some_list]