DH mentions it, assuming I was also on the WhatsApp group (I’m not) and when it turns out I’m not on the group - he takes it as I’m not invited, so he’s going for lunch not me.
Was the group created just for this catchup? You might not have been added because he might assume that adding your DH automatically includes you.
Can you contact him and clarify?
If the friend says you are definitely invited, I would be tempted to go, turning up early, and sit myself between 2 people I knew, so you didn't have to sit next to DH, and see the look on his face when he arrived.
Many years ago DH's friend (from high school & Uni) tried to contact us to invite us to his 21st, which was a night out at a restaurant. He called the house - and our housemate answered (another of DH's friends, this housemate only met DH and the 21st friend at Uni) and he invited all 3 of us - except the housemate didn't pass the message onto DH and I, and he purposely kept quiet for the next week - housemate didn't like me, so I think he didn't want me to come along.
On the morning of the party, housemate said he was going out that night, but didn't say what it was for (which was unusual). He left the house early that morning (also unusual - but it was to buy a present).
After he'd left, DH got a phone call from 21st birthday boy - as we hadn't RSVP'd to his party - this was the first DH and I had heard of it, and DH said we'd both be there.
I asked DH not to mention this to our housemate - to see what would happen if he returned later that day before the party and it took a lot of convincing to get DH to play along.
Friend came home in the afternoon with a shopping bag, and went and hid in his room. He went out again - so DH and I then got ready, but when we heard his car come back, we quickly put our dressing gowns on so it looked like we were going to cuddle up on the couch to watch a movie.
He left again - all dressed up. DH and I finished getting ready, then went to the restuarant - the look on our housemate's face was priceless - so worth it!