Depending on how you are planning on using Salesforce Ideas, your community might be a mixture of several different types of users. Here are three examples.   

Example 1: Share Sales Advice and Best Practices
If your sales team is already using Salesforce CRM (Professional, Enterprise, or Unlimited), Salesforce Ideas is included. All you have to do is add the Ideas tab to their view and you're off and running.

Example 2: Company Wide Community to Drive Innovation
Many companies want to create communities which are accessible to all their employees, across all departments. It might be that you already have some CRM users who have access to Ideas. For all your other employees you can add Ideas Only Users which are $5 per month. This license includes the home tab, the ideas tab, 3 custom tabs for things like Ideas in Action, reports, and dashboards.

Example 3: Customer Communities to Capture Feedback and Manage Discussions
Lots of companies want to use Salesforce Ideas to create customer communities similar to Dell Ideastorm or MyStarbucks Idea. The community might be used to capture feedback, or it might be a place for customers to post questions and form discussions.

With these types of communities a named user model isn't always appropriate, so we offer a login based licensing model which is similar to a cell phone plan. There are a number of different plans to choose from ranging from 250 logins a month, all the way up to 5 million logins, which for all intensive purposes is unlimited.