At times, you may need to invite external users to your SharePoint site. You can share the whole site, or just a part of the site.
The first thing you need to make sure that the site collection is enabled for Sharing. By default, it should be enabled, but we want to make sure. Login to the site as a Global Admin, and click on Admin over on the top right, then click SharePoint.
From the SharePoint Admin Center, put a check mark next to the site collection you want to share, and click the Sharing button on the ribbon
When the Sharing window pops up, you have 3 choices, you can select the choice you want, and click Save. You have just enabled Sharing for that site collection. Going forward everything is done from within the SharePoint site.
Navigate back to your Sharepoint site you want to share, and up on the top right, you will see a button that says "Share". Click that Share button.
Once you click Share, you will get a pop up window that you will have to fill out. You put the users email address in and then you want to click on Show Options if you want to change what group you want to put this user in. This is very important because you are giving an outside user the same rights as an internal user, so, they can have edit, design or any other permission you would give any other user, so make sure you select the correct group to put them in. Once you have selected the group, his save. This will send a user an email.
The user will get an email from Microsoft Online, inviting them to the SharePoint site. The user just clicks the link in the email