If you upload the .csv via the web interface, then we assume you are uploading them to the group that you chose to upload members too.
If you upload the .csv via SFTP without any group ID, then we add the user to your network but not to any groups. This means that the user will not receive any messages since they are not a part of a group.
We do not recommend uploading users via .csv's without group IDs.
But in the case where someone is uploaded without a group assignment, the user will have the ability to log into their preference page and join any public group if they so desire.