I would suggest make a SQL query for this.
SELECT
core_members.member_group_id,
core_members.`name`,
core_members.member_id,
core_members.mgroup_others
FROM
core_members
WHERE
core_members.member_group_id = 3
Replace 3 with the group ID of members (normally 3 is Members)
When running this query it will show you a list of all members.
the mgroup_others is the secondary group.
So here you then could Change member_group_id to their new id and add 3 as their mgroup_others (to give them secondary)