You ever experienced you are on flickr finding an interesting group and the only thing that show is the delighting message "This group is not available for you."?
Users registered from Europe will experience this quiet often, even their own profile settings are "safe & moderate" and should match the groups. No matter how you'll change your profile settings, even if the group is supposedly set to "content for all", this is in fact a group admin only can help you with.
User settings: account > privacy & permissions
If you're an admin of a flickr-group and want your group be accessible for all, you should go and check for two things:
1. "Your Group" > Administration > Moderating Your Group >
2. the NEW hidden feature "18+". It is enabled by default.
As I am a user from the EU, this feature option seems not to be available for me, but I was told, that you'll find it following the path 
Administration >"About Your Group" section, at the very bottom.
I got a *NEW* "Restricted Content", but no "18+" filter showing (pic below)
I assume, that non-EU users have either another *NEW* or under "Restricted Content" something about "18+".

If there is anyone that could provide me with a picture of that specific NEW setting causing the problem of viewing groups to others, I'd be happy to add it to the post.

Meanwhile I hope, this little guide will help figuring out the solution of non-accessible groups and bring plenty of new members to you. 

Happy sharing!