Online group signup
If you want to allow people to join your group directly from the web then enable ‘Online signups’.
This will enable a public web page which you can share amongst potential members to sign up. You can also host a sign-up form on your own website by following instructions on the bottom link.
Requiring New Member Approval
You can also enable the "New members require administrator approval" setting which will place anyone signing up in a holding area where the group administrators can then go and either approve or reject them. With this setting enabled, administrators will be notified by email when someone signs up.