To accept a shared album invite, follow these steps generally for iPhone users:

1. Open the Photos app.
2. Tap the "For You" tab at the bottom of the screen.
3. At the top in the Shared Album Activity, you should see the invite notification.
4. Tap the shared album invite.
5. Tap "Accept" to join the album.

If you don't see the invite:

• Ensure Shared Albums is enabled in Settings > Photos.
• Check Notification Center or emails for the invite.
• Ask the sender to resend the invite if needed.

Once accepted, you can view, like, comment, and add photos if allowed. On Mac, open Photos, go to Shared Albums, and accept from there. On Android with Google Photos, check the Sharing tab for invites and tap Join. This covers the main ways to accept shared album invites on iPhone, Mac, and Android.