It is depend solely on your visitor numbers when it comes to choose hosting for forum type website. All shared hosting even Hostgator is still has its own limited on load capability toward this type of website. It is also depend on the forum software you are using, or going to use. vBulletin is ok while phpbb is a resource hog and they both do consume different amount of resources even the similar number of online users. Around 50-200 active users daily you may still with most shared hosting. This list may help you. If you have more than that you may consider to sign up on VPS / Dedicated server instead.
How much your unique visitors daily really? Only ten signup a day is not a big deal. Shared hosting still capable on it. I do recommend inmotion or hostgator for a quality web hosting and not oversold.