who can see it?

[shauna]

I was added to a private group. Can anyone outside the group see I was added to it on their timeline on Facebook?

[Andrew]

No...for private groups no one from other peoples cannot see only public groups can be seen.

[Trojan Master]

Only if that friend does not have his privacy settings set to private and if he or she decides not to share it with others. Bottom line is that it is easy to see you stuff through others peoples facebook