How can I share an iCloud photo album?
You will need to create the album with iCloud Photo Sharing in order to do that. iCloud will not let you share a personal album that you created as they are only a means of organizing media. 

With iCloud Photo Sharing, you can easily accomplish your goal and share the album either with specific email addresses or you can share a link, if you make it public. 

I suggest selecting all of the photos in the album that you have already created and choosing to share them to the iCloud Photo Sharing album that you create in order to publish those photos.