Many Invisalign patients have attachments on their teeth to allow extra grip for the aligners and to help the teeth move better. These attachments are in the form of a tooth-coloured dental bonding material that are cemented on the teeth, and may come in various shapes (rectangular, round, etc). Where these attachments are placed would depend on their purpose and the type of tooth movement your orthodontist would like to achieve.
All of my Invisalign patients have attachments, some more than others, so not to worry. They are usually small in size and rarely cause discomfort. If t your orthodontist will be able to smoothen or trim it down for you.
Hope this helps, all the best for your Invisalign treatment.