Similiar to category drop down box for a list of terms with the same parent, I want to make such a drop down for a list of tags with the same group name( or some other conditions that works). Seems WP doesn't provide a way to use term group, any other way to make the drop down ?
The examples i quote in my answer can be found here, i also recommend you read that page carefully to help you with this.
But lets grab an existing example there and change it a bit.
I hope this answers your question.