ようではないか

Shall we
Plain Volitional + ではないか

Volitional + じゃないか/ではないか is just another way of asking someone if they want to do something.
Its equivalent to ようか/ませんか

It’s stronger than just ようか, and tends to be used in things like speeches or calls to action.
As ではありませんか in particular, it tends to be more “formal” than ようか.

Examples

• 皆で宴をしようじゃないか
  Lets all have a celebration
  
  
• 先生も誘おうではないか
  Lets also invite the teacher.
  
  
• 一緒に政治の腐敗を止めようではないか
  Lets stop political corruption together.
• そろそろ会議を始めようではありませんか
  Shall we go ahead and start the meeting soon?

References/Additional Reading

Edewakaru (Double check for nuance)
Kyoushi.net (Double check on nuance)