I was the only one not invited aibu?

[breadandwines]

I started a new job in July in a call centre.
I quickly made friends with my team.
In august I started seeing a man off our team and was invited on meals out (few and far between with covid ) but we had outdoor meet ups in between.
We all speak on a group chat on WhatsApp.
One of the group doesn't like me(it's obvious ) and I think she doesn't like the fact I'm part of the group.
Nye the girl wrote to one of the members "sleep well,excited for tomorrow "
I didn't think much of it and thought maybe they were meeting for a coffee or something.
Anyway last night they had a house party (which I wouldn't have went to anyway due to covid restrictions"
They didn't invite me and the guy I'm seeing is there and didn't invite me.
He hasn't spoke all day yesterday or today
Aibu to feel pissed off ?

Honestly OP I think you're the one being unreasonable.

A guy you've been casually dating for a few months wanted to go out with his best mates who he's known for years, and didn't invite you.

That's pretty normal.

The fact you work with his oldest friends complicates matters but you barely know them and the presence of a near-stranger in a group of old mates does change the dynamic.

If I was meeting my old uni friends who I've been best mates with for 20 years for drinks it wouldn't even occur to me to invite a new boyfriend. I'd be pretty upset if some guy I'd been casually dating for a few months kicked off about being excluded and not wanting me to spend time with my oldest friends without him.

It's a shame your co-workers aren't more friendly, but from your OP it sounds like they are friendly to you, it's just when they have meetups that are for the "old gang" who have known each other for years, newbies aren't invited?

It sounds like you regard the relationship as being more serious than the bloke does. Five months is nothing. Stop seeing him.