can i share an existing iphoto album with someone new, without having to create a new album?

If you mean can you add people to a Shared album that you have created in the Photos app, then yes, you can add additional people.

Open Photos

Click on the Shared Album that you created under the Activity header

Click on the little person in the upper right-hand corner:

Then click on the "+" in the dialogue box that comes up to display your Contacts and select the new person:

Cheers,

GB

You can invite additional people to your shared album, yes. But be considerate, if you do that. I would not do it, if you set up the shared album in a way, that allows the subscribers to add photos to your shared album. If the subscribers already added added photos, ask the current subscribers for permission to share the photos with new subscribers. If you are not sure, if the current subscribers will want to share the photos they posted with an additional subscriber, it would be more considerate to create a new album for sharing with additional subscribers.

are you actually using iPhoto as you stated and where you posted? Or are you using Photos, a new and totally different program

LN