The Vましょう form is used to make suggestions or proposals to do something together. Use of the phrase いっしょに(together) is not necessarily required for this meaning, but it clarifies the speaker's intention to do something together with the person spoken to.
たべます します |
→ → |
たべましょう しましょう |
いっしょに あの みせへ いきましょう。
Let's go to that store together.