You need to give a user permission to a SharePoint Site in Office365.
This is done using the Office 365 administrative console. You need to be an Office 365 administrator for this.
- Login to Office365 as a user with admin permissions in SharePoint and open the site you need to update.
- Click the “Site Actions” button at the top left to get a list of changes you can make; click on “Site Settings”
- In the main body of the page, click the “Site Permissions” link under the “Users and Permissions” header.
- You will be sent to a page with all the current access permissions listed. Click the “Grant Permissions” button in the toolbar.
- Select the user you wish to give access to (you can use the small icon that looks like an open book at the bottom, right of the user list to browse your Office365 users to find the one you need). Select the permission level the user needs and click OK. By default, SP will send a welcome message to the user to notify them of the permission change but you can disable that by unchecking the “Send welcome e-mail” box in the third section of the form.