Go to Team Site
->Site Collection Administration
->Site collection features
->External user invitations
->Activate (Allow external users to be invited to the site).
Then when I back to Site Actions ->Share Site
If the pop-out windows still shows “Invitations to users outside your organization are currently disabled” follow below advice.
Resolving this issue depends on what type of subscription you have. If you have a Small Business (P1) subscription, you can deactivate and then re-active the feature and it should resolve the issue. If you have an Enterprise (E) subscription, there will be an additional area where you will need to make sure the feature is enabled. First, you will need to log into the Administration Portal. In the Administration Portal, you will need to click on “Manage” under SharePoint Online. When the next page loads, click on “Manage Site Collection” and your list of site collections will load. Click on the “Settings” button at the top and you will have the option to “Manage External Users”. You will need to set the radio button to allow the external users. Save the changes and then try again.