This place holds almost every day of the week an event, musical, dramatic, pictorial, vocal, Poetical .... It's a association, so i think you have to pay a little something every year, and you must be a member or friend of a member to get in, but you can go just like that, i don't think the boss will have a problem, he might ask you after to become a member.
For more information, here is the website address: www.orpheu-negro.org
Tel: 0030 6947687495 Address: 49 Mykalis and Paramithias, Keramikos, Athens.
Located in Keramikos (between Metaxourgio and Gazi) Orpheu Negro is an association supporting foreigners artists living in Greece by offering them the opportunity to make concert and exhibition in a friendly place.