[psychonic]

That's the internal Steam function for setting the search key and group data.

Normally, the engine limits the output to 1070 bytes due to limitations with the legacy master server, which is no longer relevant. The function and Steam backend do allow up to 2048 characters for this (with each group taking up five characters, plus the length of the group id, plus one for the comma on all but one). Ex. "grp:123456i,grp:123457i,grp:123458i,key:myse archkey2151".

Calling it directly bypasses the engine's limit, however it would be tricky to do directly from Sourcepawn since the SteamWorks extension does not expose it (but it would be trivial to add to that).

With luck, they'll eventually just remove that panel that shows group servers. The implementation is awful, as this thread proves.