This update addresses all known issues with the houses system. There are also a few new features added.

Invitations have been made more flexible. You can now invite your friends before even entering the house.
If you do so, the house will be infested with server zombies instead of client so you can clear the house together with your friends.
Stealth mode is active for those zombies.
When you stand up on the entrance marker, a house panel will appear. It will let you either enter the house or invite someone.

Changelog:
Added house panel.
Houses can now be infested by server zombies.
Fixed all known houses which spawn you into the void.
Weapon pickups no longer replace your ammo if you already have more.
Fixed some non working pickups and added new ones.
Fixed a bug which would leave you in the house without an exit if you reconnect while inside it (thanks to Ulisses for reporting)