I already suggested that the friend sharing the planet has to guard the planet once it's colonized by the friend he wanted to, to avoid another legion member attacking and taking the planet ...
Cause anyways, some legion members will eventually get that planet and fux it if it's worth the trouble. Like for instance a post I saw about a nice exotic planet shared with a help attack/don't invade message ... That was actually invaded haha
that's what happens I guess when you add people that you don't really know ...
When I check a shared planet, I go to the friend's profile and I check if the current owner is in his friend list ... Avoids troubles as well ... Well, with the new feature I guess it won't be necessary anymore.