I highly suggest you spend 5 minutes to register for Verified Membership if you intend to play with us in the future. Don't worry it's completely free, and it should protect you from donor slot kick. It's not 100% immunity, but it's much higher than regular players.
To become a VM, just click this link:
http://hyperiongaming.org/index.php?page=page3457and follow the instructions on the page.
You've already done all the work. You simply need to join our public Steam group (
http://steamcommunity.com/groups/hyperiongaming ) and then enter your SteamID and click Submit on that form. Let me know if you need help.
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As for kicking to make room, there are a lot of different ways to handle donor slots, and all have positives and negatives. Our current system is far from perfect as you mentioned, but for people playing frequently with us, the VM (and becoming a donator obviously) system should help a lot.