This article explains the difference between inviting and adding members to a Yahoo! group.

The main difference is that invitations require a response before a member is added to the group.

Also, users who are "added" rather than "invited" will be treated as "email-only members" of the group. This means they won’t be able to use the group's web features (photos, files, calendar, etc.) until web access membership has been completed.

The "Invite Members" feature allows you to send an email invitation to people to join the group. You can invite people by email address or Yahoo! ID. These people won’t be added to the group until they have responded to the invitation.

Note: The "Add Members" feature that allowed adding up to 10 new group members in a single session has been discontinued for newly created groups as of April 11, 2012. This feature will remain available for groups created prior to April 11, 2012.

Additional Note: It’s against the Yahoo! Groups Community Guidelines to add people to your group without their permission. Yahoo! Groups provides the "Add Members" feature as a convenience to group moderators for those occasions that require directly importing a few email addresses to a group. If you abuse this feature, your group will be deleted and your Yahoo! account will be suspended.