Modifying Organic Group subscription to save data
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I am using organic groups in drupal 6 as user driven social events. I have modified the content type so that the organizer (group admin) can specify which days the events take place on i.e. Mon, Tues, Thurs, Sat.

Each event can only accomodate a certain amount of people. The assumption is that not all people will attend every night, so I would like for subscribers to specifiy which of the group days they will be attending the event i.e. Jane -> Mon, Sat and Mark -> Tues, Thurs, Sat. This is where I am getting stuck: How can I alter the group subscribtion form from my custom module to add in extra checkboxes with the days that can be selected, and save the selections for later reference in an attendance calendar.  