2
After you have set you block to 'visible'
1- make a note of the name of your block title.
2- Go to sytem admin>groups>edit groups registered users
3- Find your custom block and make sure it has a check mark
4- Update the group
5- return to groups main page
6- edit groups anonymous users
7- Find your custom block
8- Make sure there is a check mark against it.
9- update the group.
10 -repeat for any other groups you wish to see the block
Job done