Due to complaints about decreased performance, we asked Valve to disable Steam Overlay for ZK by default.
Unfortunately this takes out parts of the invite functionality.
(erroneous info below compacted)[Spoiler]
The crux of the problem is that for the Overlay to work properly, the Steam API thing needs to be called by the same process that does the heavy graphics work - in this case, Spring. However, Spring can't do this as-is, so instead, in case of ZK, the Steam API is called by the Chobby's .NET based wrapper program.
A proper fix will require an engine update, and the last time a new engine was tested for ZK it had issues (broken terraform, etc).
So three things need to be done to solve both the overlay performance regression problem and broken invites:
1) Implement a way for Spring to call the SteamAPI directly rather than via wrapper program. While somehow dodging all the kinds of nasty that the infectious GPL license and engine developers' sense of squeamishness may say about linking proprietary code, then get it merged into the maintanance branch.
2) Test the resulting build that contains the fix for presence of unrelated regressions and general fitness to run ZK, and obtain fixes to issues that are found.
3) Apply new engine to ZK and modify the game and its lobby to use the new way of accessing Steam API.