Yet another Naples question - How do I join the HRE? Do I need to move my capital to a province in the HRE and buddy-buddy with Austria or what?
You can't change your capital to a province that is already in the HRE.
Instead, your existing capital needs to border HRE land, then you click Add to HRE on that capital. So, move your capital first (if necessary to get it bordering), then add it to the HRE.
You can only add your capital if the total sum of your per-province base tax is 50 or less. So look at all your provinces, and add up their raw Base Tax number (as shown in the province interface). This is not modified by buildings, tech, anything, it's the basic Base Tax number.
You then need relations with the emperor of 100 + (2 * SumOfProvinceBaseTax). So if you were an OPM with a Base Tax 1 province, you'd need relations of 102. If you have the maximum sum of 50 Base Tax, you need 200 relations.
If you have more than a total of 50 Base Tax, then the add to HRE button will be greyed out with a message "We are too big, the current Emperor would never allow it."