To begin, log into your Office 365 site using an Admin account.
From there, open the Settings menu. Select Site Settings.
Next, under Users and Permissions, click on People and groups.
Select the Group name to which you would like to add a user. From that group’s page, click on the New (+) icon.
A pop-up will appear asking you to share the site with the user. Enter the name(s) of the user(s) you would like to add to the group (for more on how to create a new user, see Creating New Users on SharePoint Online, Part 1: How to Create a New User Click ). Once you have entered in the name(s) of the user(s) whom you would like to add, click on the Share button.
Once the user has been successfully added to the group, you will see their name listed under the Group name. To verify that the user has been added, have him log into the site and try to access the group.