Group owners can make private and secret groups accessible to outside contributors if this feature is enabled in your community (by your community administrator).
When enabled, private and secret groups can become externally accessible. When a group becomes externally accessible, group members, both external contributors and standard community users, can invite external contributors to these groups.
When disabled after being enabled for a period, all content created by external contributors will continue to exist, and external contributors will continue to be visible in the community, but they will no longer be able to log in.
By default, externally accessible groups are enabled in internal communities (typically those that serve employees) and disabled in external communities (typically those that serve customers, vendors, and partners).