Only registered members can add a topic and they can only create one within an existing forum/category.
Here's how they do it:
First of all they must navigate to the Forum on your website and then log-in using their existing log-in credentials..
Once logged in, they can choose which forum they would like to add a topic to by clicking on the relevant Forum title:
From here they can add a new topic by clicking 'Add Topic'
NOTE: Topics can only be created in existing forums. Members are unable to create Forums orCategories themselves.