At last I found how to do that.

For displaying a group in the registration form, after enable "show on registration form" option in the "admin/config/people/accounts/fields/og_user_node" page, you should:

1. Add the group register field to the group fields on the "groups field setting page"
2. Then you should check the "Show on registration page" option on the group editing page.

At last I found how to do that.

For displaying a group in the registration form, after enable "show on registration form" option in the admin/config/people/accounts/fields/og_user_node page, you should:

1. Add the group register field to the group fields on the "groups field setting page"
2. Then you should check the "Show on registration page" option on the group editing page

At last I found how to do that. For displaying a group in the registration form, after enable "show on registration form" option in the "admin/config/people/accounts/fields/og_user_node" page, you should add the group register field to the group fields on the "groups field setting page", then you should check the "Show on registration page" option on the group editing page

At last I found how to do that. 

For displaying a group in the registration form, after enable "show on registration form" option in the "admin/config/people/accounts/fields/og_user_node" page, you should:

1. Add the group register field to the group fields on the "groups field setting page"
2. Then you should check the "Show on registration page" option on the group editing page.