Inviting your members during the Add Members process
You can invite members to join your group by using the built in invitation message. When adding members toggle on the Send an invite option:
Customize your default invitation text
You can customize your message before adding your members in the Membership Emails section in your Group Settings area.
Inviting your members after the Add Members process
Otherwise, if you've already added your members and you want to invite them after you've set up your group, go to your Member Database and select all or specific members you want to invite.
On the bottom left, click the Re-send Invite button to review your invitation text first, before you send it.
Customize the text if needed then click Send Invitation:
The invitation message looks like this for your members in their inbox: