Who approves membership of a multi-company group on Workplace?
Multi-company group (MCG) admins must approve new members of multi-company groups by default.
Keep in mind that multi-company group admins can turn off admin-approvals for new members in the group's settings. If the setting is turned off, anyone can add new members to the group without admin approval.
If the group is a closed multi-company group and the admin disables approvals for new members, then anyone with a group invite link will be able to join the group without admin approval.