Groups are private, so you will need to be invited in order to join. The group creator can invite you to their group either by sending you an email via Memrise, or by sharing a direct link to the group with you.
Please note: Group functionalities are only accessible on the website at the moment.
If you are not receiving the invitation email, please double-check your spam, junk and social folders.
Simply click on the group's link while you are logged in with your Memrise details on the website, and you will be automatically added to the group.