If it is a big group of students, you can either ask them to enroll themselves in the group (keeping open membership) or invite them to join the group (by invitation only).
In the latest version of Mahara,-1.1 - you can create a "Controlled membership" group which allows you to enroll students in a particular group and "lock" that membeship. However, if you are using lower versions of Mahara, this option doesn't work,
Regards
]]>Have you created a group? As Nigel mentioned, forums exist within groups. If necessary, you can create groups to which all users are automatically added when they join your site.
Once you've created a group, you should find it's quite straightforward. Sing out if you get stuck.
Myles
Forums are attached to groups, and have been standard in Mahara since 1.0.