Do any of the following:
Add a comment: Type a comment in the comment box.
Share Live Photos: To let viewers play a Live Photo, select the Live Photo checkbox. Deselect the Live Photo checkbox to share the Live Photo as a still image only.
Add the selected photos to an existing shared album: Click a shared album name.
Add the selected photos to a new shared album: Click New Shared Album, enter an album name, click the Plus button and invite others to see your shared album, enter a comment if you want, then click Create.
In the Photos app View photos shared with you in Photos on Mac