It's a good idea to have an event manager. When I was in Sydney, most of the hostels had a dedicated event person, usually a receptionist. They would liaise with the nightclubs and bars in the area, along with rounding the guests up, and taking them out to the bars and clubs. We would go around and knock on all the doors (more like me) and ask who was coming, and if they weren't coming, why they weren't coming!!! I dragged so many people out when they were in their bed or PJs... :)
I'm not sure if you have any longterm guests staying at the hostel, but it might be worth asking if they want the job? I would imagine you don't need a fulltime event manager, just one in the afternoon/evenings. If the event manager is a guest, I would give them free accommodation and some pocket money for taking the guests out. The bars will usually give the events manager free alcohol as well. It needs to be someone who can organise people, and be a lot of fun with guests. Enthusiasm and people-organisation skills are the most important thing for that kind of role.
You could also make the role full-time by making them assist with breakfast in the morning. They would be there to monitor breakfast supplies, washing up (bowls, plates, cutlery, etc) and clean up any spillages the guests make. It will make the hostel look very professional having someone dedicated at breakfast.
With your volunteers, they should be assigned to someone in the area they are working in. For example, when you say organising the common room, you mean clean it, that volunteer should work with the person in charge of cleaning. Same for recycling, the recycler should work with the person in charge of cleaning.
Can you expand on what all the volunteers do?