No - applicants are not automatically added to your network when they apply for a job at your company.
You can choose to move candidates into your network via the applications page though.
1. Head to Jobs > Manage Applications
2. Select the job you are managing
3. Use the tick boxes along the left to select those you want to add into your network
4. Open the 'with selected' menu at the top and click 'Add to list'
5. Select an existing list within your network or create a new one to add your candidates too
Adding those people into your network will help you save time on future recruitment for similar jobs, allowing you to quickly find and contact people who've already expressed an interest in your company.