I would leave them rare materials or dyes in chests and see what they would come up with.
The fun part was that we didn't have to play at the same time, every now and then I would log in and they'd have left something in my house to let me know they found my hide out.
We eventually invited some of their friends and a co-worker's kid who was getting griefed in public servers.
Every so often we'll spend an evening on the couch playing some co-op or competitive game but we don't spend much time in our Terraria world anymore.
I should check it out and see what's become of it.