HD-MKV has announced that they will no longer be giving out invites. Below is a PM quote from a system message:

You have received 6 Invitesplease invite your friends and family. once these invites are gone the only way to receive more is become LIFETIME V.I.P. or a staff member invites someone

Here is a more detailed description (taken from their news):

The staff of HD-MKV have decided that there should be some changes to the invite system here. We feel that currently a smaller active community is what we need here. After the current invites are gone, all users will have no new invites. Invites will be closed for all users except Lifetime VIP, VIP, mods, admin, uploaders, Internal encoders and staff. Said userclasses will have unlimited inivites to offer elsewhere to trusted users.
We understand that you may want to invite your friend in the future after the invite system closes and we will have a system in place to consider user requests.