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What is the 'Group Field Title' for?

The group field title is the title that will be displayed right above the text box where the group will enter in their complete Facebook URL: http://www.facebook.com/username.

Will the 'Mirror Profile Field Title' also be displayed above the text box where the member will enter in their Facebook name.?

No, the text box will use the title you gave your Xprofile field as described in step 6 of the plugin installation instructions. The 'Mirror Profile Field Title' is used to fetch the members meta ( attached data ) though if the two differ, no data will be returned thus no button will display.

More Information

More informations (in portuguese): http://www.artewebdesign.com.br/plugin-para-wordpress-buddypress-facebook/