Chinese participants at a conference in Vienna

We plan a conference for European and Asian participants in October 2012 at a hotel in Vienna and we want to invite some of our Chinese colleagues. We arrange the conference from Copenhagen, Denmark, but the meeting is held in Vienna, so they have to apply for visas to Austria, but do they need a letter of invitation from us, the arranging party? Thanks, in advance for your reply!
Regards,
Ms. Aase Naess
Localization Coordinator
SAS Institute