You can complete a contact teacher form with a teacher, then message him or her regarding your request for a lesson invitation.
Lesson invitations can only be sent by teachers. They can be sent up to 15 minutes before the class starts. For example, if you ask for a lesson at 9:14, presuming both teacher and student read the message at that time, the teacher can send you a lesson invitation for a lesson starting at 9:30.
Hope that clears it up for you. Please check out my teacher profile and feel free to book a lesson :)