I saw a thread last night about some guys asking if there was a plugin to add a additional field to group. I wrote this plugin which im using on my site. All you have to do is change 'Trading hours' to what ever field you would need.
It adds the form to group creation - step one and saves it in group meta.
Save the code as bp-add-field.php in a folder called buddypress-additional-field.
Hope it will be helpfull.