iCloud Photo Sharing - Not receiving invitation
I've never experienced a similar issue, but these 2 steps won't hurt:


Make sure both your iPhone and the other member's iPhone have iCloud Photo Sharing AND iCloud Photo Library turned on.
Make sure you have the same settings set up on your Mac/iPad if you use any.


Also, if your shared iCloud Photo Album has more than 5,000 photos, you will need to make a new album to continue sharing.

Here's a great site for everything related to iCloud Photo Library:
https://tidbits.com/article/15584