Do not specify a topic_id when creating a topic:
curl -X POST "https://my.discourse.com/posts" \
-H "Content-Type: multipart/form-data;" \
-F "api_key=theGeneratedAPIKeyForanAdmin" \
-F "api_username=anAdmin" \
-F "title=This is an API created topic" \
-F "raw=This is the end of the world as we know it, and I feel fine."
When creating a group be sure to use the id of the group and not the name of the group and to use a PUT request:
curl -X PUT "https://my.discourse.com/groups/group_id/members.json" \
-H "Content-Type: multipart/form-data;" \
-F "api_key=theGeneratedAPIKeyForUserNamedanAdmin" \
-F "api_username=anAdmin" \
-F "usernames=newForumUser"