NAT is Open but can't join multiplayer games or parties

No the issue hasn’t been and it isn’t my nat type sure my nat type is moderate but allows me to join players and those players can still find a lobby

It definitely could still be your moderate nat type.

With a moderate nat type youd be able to join other specific players who have an open or moderate nat type and join games if that other person is the host of the party but you could run into issues finding matches when you are by yourself.

Per: https://support.xbox.com/en-US/help...erate NAT: You might not,the host of a match.

Open NAT: You're able to play and host multiplayer games with people who have any NAT type on their network.

Moderate NAT: You might not be able to play multiplayer games with some people, and normally you won't be chosen as the host of a match.

Strict NAT: You're only able to play multiplayer games with people who have an OPEN NAT type. You can't be chosen as the host of a match.

Hmm.. Seems like a NAT type issue.

When sitting at the MW3 Main Menu you should see "NAT Type:". It will either be Open(Good), Moderate(ehhhh), Strict(Not so good).

If it is either "Moderate" or "Strict" that could be the reason why you can't connect to your friends. You can figure out how to make yours "Open" by searching the forums for this sticky post by ArminatorX... How To: Open NAT to resolve Party and Multiplayer
problems. Once you get your NAT type to "Open" you should be good to go.

Just in case that NAT is not the problem make sure that your Parental and Online Safety Settings allow you to join parties.

I have a question what could cause this issue if both me and my friend have a open nat type and still can't join each others live party and game

I have the exact same problem, NAT types wide open, can connect to other friends party but when we want to creat a party for the two of us to join we can't connect to the party!

Tried checking your NAT Type? Should be open Nat otherwise you wont be able to join multiplayer games etc