SharePoint on Office365 allows 50 extranet users by default. What is an extranet user? Basically they are users that can that can have access to a SharePoint site (i.e. subsite only) without the need to have an Office365 license, however they are a full user of that site just like a standard Office365 user. The restriction is, that they are limited to accessing just that site.
These extranet users are not enabled by default, so the first thing you will need to do after you have created your SharePoint site is to go into the Office365 admin console for SharePoint and select Settings option and then Manage External Users from the sub menu as shown above.
You will then need to select the option to Allow as shown above and Save the settings.
You then need to go into the Site Actions, Site Settings then select Site Collection Features from under the Site Collection Administration section. One of the options is External user invitations as shown above. If that features is not Active then press the Activate button to enable it.
To invite an externals user now select Site Actions again and select Share Site from the menu that appears.
Now simply enter the email addresses into the desire site groups and press the Share button.
If you haven’t correctly enabled Extranet users you will see a message displayed in this box as shown above telling you that “Invitations to users outside your organization are currently disabled”. If so check the above enabling steps.
If the invitation is sent successfully you should see a message like that shown above indicating that the users will be added to the site once they accept the invitation.
The external user should receive an email like shown above inviting them to join the SharePoint site.
The link will take them to a page where they can select to either connect using a Windows Live Id or a standard Office365 login. If the user doesn’t already have a Windows Live Id they will be able to create one via a link shown on the above page.
Once they have logged in they will have access to the SharePoint site.
If you now look at the securities for that group which you added the external user to you will find them listed by the email address as shown above.
They will now be treated as a normal SharePoint user for that site.
Can you have more than 50 extranet users? Yes, but you need to purchase an additional license for each one after the first 50 I believe.