Looks like Email is all you have.

However, you could try the General Access settings so if you share 1 of the interfaces by link from the General Access, they should in theory get all of the General Access interfaces.

Only if that is an option for you of course.

Does the copy link at the bottom right-hand corner when you've selected multiple not work? My assumption would be that you could send your own email/slack/etc. with the link you copy and as long as General Access is set to the group you want, it should work in theory. 

I'm not 100% positive there since I've never used invite by link, so I don't know the features.

@Glen_Bayless will test it later in the afternoon as soon as I have guinea pig at hand. 

@Kyrstin_Graves will test it as well later. Haven't tried/noticed it yet to be honest. 
What confuses me is that this is just the link for "invite by email". 
However, with one Interface and "Invite by link" I can also setup the rights (commenter / editor etc). 
I can't do this with the "Copy Link" button.
The link itself is also always the same, doesn't matter which interfaces I select. 
However, this is where maybe the "General Access", mentioned by @Glen_Bayless, comes into play. 

From the documentation regarding General Access: 

---

The general access section allows interface builders to set more global, over-arching rules for which visitors can access one or more interfaces.

I will provide an update as soon as I tested it later. 

I tried both methods and still just one interface is shared 😞 
So I had to send over two links to get them access to both. 

Bummer! Thanks for the update.