By access to data if you mean accessing data from within the Hub Site/Initiative, then this might help;
However in my experience, in order to share data to open data site/initiative it should be shared publicly hich is then shared with Open Data dedicated group. Which makes it available for public (someone knows how it can be searched and accessed).
Sorry forgot about Index Data button. So Open Data registers all the items shared with all the open data groups in the organization and groups taken from outside the organization. When it does so, it builds an Index which updates automatically whenever an Item is shared with or removed from Open data group. If for some reason this index is not updated and you can not see the changes made (Sharing or unsharing an Item) in Data page or in Hub Site the you can rebuild the index manually by hitting the Index data button.