WordPress.com is a blogging host. It's not a wiki or forum provider.
Online forum - An Internet forum, or message board, is an online discussion site where people can hold conversations in the form of posted messages.
Wiki - A Web site developed collaboratively by a community of users, allowing any user to add and edit content.
You can get a free forum or wiki on the internet and link to it from your WordPress.com blog.
Here you can use the P2 theme which is Twitter like. http://theme.wordpress.com/themes/p2/
user roles > http://en.support.wordpress.com/user-roles/
adding users > http://en.support.wordpress.com/adding-users/