Thanks to Giuseppe Amato the OpenId Connect authenticator have two new features: add prefix for group names and support for groups returned as comma delimited string from id provider.
See https://extensions.xwiki.org/xwiki/bin/view/Extension/OpenID%20Connect/OpenID%20Connect%20Authenticator/#Hxwiki.properties for more details (properties oidc.groups.prefix and oidc.groups.separator).