I am thinking to order a dedicated server at RS or any other companies which offer similar price for backup purpose. I would like to share this server with other people. Let say 10, so everyone will get around 5gig disk space and 40 - 50 gb transfer. The monthly fee is around $10 for each person.
Becasue this is backup server, so I think no http access is necessary. This can reduce unnessary traffic and fair for share users.
Don't you think is there any people interested in it? Or is there any problem that might encouter?
Normally there is a number of issues on sharing server. You can search the forum archive for some discussions on it. (including comments made by me).
However, because you are offering backup space and (presumably) only FTP access, the idea is actually very doable, and avoid almost all the issues that comes with sharing server, since you don't provide root access.
The only problem is if you can get enough people to justify the cost. Also, you will want to make clear that you don't provide root access (or any other form of access except FTP/NFS/rsync).
Lastly, don't forget that by doing so you are not just "sharing a server," you are also providing a service, so you are responsible for server up/downtime, technical support and any other things a service provider need to provide. So don't forget to factor in those costs as well. It isn't a completely automated process (although it is close).