How to allow online group signups
You can allow people to join your group through a public website page or an online form.
Login, click the group and click Settings in the sidebar:
Scroll to Customise and click Allow online signups:
You can either copy and paste a link to a page for people to signup (1) or copy and paste html to host a form on a web page (2):
You can also toggle the option for New members require administrator approval. Administrators will receive an email for them to approve the signup: