are you meaning import members? you can send invites and add members but with the add feature you must have permission from each person first or risk violating TOS if one complains.
How do I invite members to my group?
How can I directly add (not invite) members to my group?
“Important Notes regarding use of this feature:

It’s against the Yahoo! Groups Terms of Service 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.
Yahoo! highly recommends using the Invite Members feature as the preferred method to add members to your group.”