To give another deviant a direct link to this FAQ answer, use the following code in your comment or forum post:
How does the 'Film' invitation process work?
If you are invited to join the Film Gallery, you will in turn be given an invitation to extend to another artist to join the gallery as well. You can only send out an invite to someone who already has a deviantART account.
In order to send out an invite visit the following the page and enter their username in the box and click "invite". You can revisit this link at any time to see if they have accepted the invitation. The person you invited will receive a note in their message centre which will contain a link to where they can accept their invitation.