1-From your Admin page, click on the icon with the 3 lines (hamburger button ☰) next to View your store,  Customers>Groups

2-Click on Create to add a new group, for example, you can name the group Newsletter, to specify that all customers inside that group will be sent newsletters.

3-Add the customers into the group you created, by going to the navigation menu, choose Customers>Customers

4-Choose a customer you want to add to the group or create a new one, then from the Actions tab, click on the edit icon, and from the drop-down menu, add the customer to the group you want to.

5-Click on save changes