How to Invite External users to your SharePoint Site
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.
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.