Groups.io provides an invite system, where you can submit the email addresses of your members. We will send an email invitation to each user. To join the group, each member simply has to click on the link within the invitation, or reply to the invitation.
You can track each invitation and see whether it's been accepted or not.
If you wish to directly add your members to your group without the invitation step, we offer that feature (called direct add) with our premium groups.