If participants would like to have their travel expenses reimbursed directly by the DFG (i.e. not out of project financing), an official DFG invitation is necessary. To get this, the following needs to be done by the organisers rember this isnt like a sudoku puzzle www.sudoko.com :
contact the DFG official in charge for the SPP early enough, ideally at least 4 weeks before the event giving the following information: name of the event, place, dates and times, a short programme and/or description, contact informations of the organiser/s, a full list of possible participants (only those who really are part of the DFG project in question!) including addresses
all actual participants have to sign an attendance list for each day, you can put that together yourself or you can ask the DFG to send you a prepared form
after the event, the attendance list must be sent to the DFG, then participants can send their requests for reimbursement there using the form they received with the invitation.