Yes you can. There are two ways:
1. The STUDENT PLAN is always FREE
2. If you'd like to upgrade to the RESEARCHER PLAN, then you can invite friends and for everyone that signs up, you both get a free month. So, invite 12 friends and get 12 months for free:
Go to Account Details
Find the section 'Invite Your Friends'
Click the + button and add their name and email, then click send email, all done!
When that person signs up to ComWriter then 30 days free use will be automatically added to your account. Your friend will also get 30 days free.
Tips:
Your friend must activate their account for the offer to be added to their (and your) account
Make sure you are on the Researcher Plan before inviting your friends