Office 365 allows you to convert an “Intranet” sharepoint site to an “Extranet” sharepoint site meaning that you can invite external users to your site.

Notice that you have a Share Site option in the Site actions:

If you select that you will see that you cannot share will external users:

So we need to activate some features to make it work…

Here are the steps to do that:

1. In the administration, go to Site Collection admin, select the site collection and then click on Manage Share By Mail:

2. Choose Allow

3. Go in the Site Collection features and activate the “External user invitation” feature:

4. Go back in the Share Site option in site settings:

5. Invite an  e mail:

and here you are…