と いっしょに indicates the person whom the subject of the sentence does something with.
わたしは おとうとと いっしょに あそびました。
I played with my younger brother.
Lesson 4-8「いっしょに あそびましょう。」
Let's play together.
Lesson 8-3「おとうとと あそびます。」
I play with my younger brother.
When suggesting doing something together, the phrase Nと is not necessary.