I just tested the most recent version from github and that seems to work fine for me.
I tested calling register_field_group in both a fuction called on the init hook, and right in a plugin file and they both seem to work fine. I think I’ll keep it in the init hook unless that’s a bad idea of some reason.
It might reduce problems down the line if you create a specific hook that people can use to call register_field_group kind of like the posts to posts plugin does for their p2p_register_connection_type and the p2p_init hook. You could wrap the exported code in a function and include the hook right there so people wouldn’t have to do it themselves.
