Friend can't join xbox live party or game invite.

I'd start by clearing the system cache (it's just updates plus other temporary files, nothing that's at all important to anyone) by:

1) Tap the silver Xbox Guide button on the controller, go right 2 clicks to Settings, pick System Settings.

2) Select Storage (or Memory).

3) Move over any hard drive or memory unit (not the Cloud) & press (Y) on your controller & pick Clear System Cache.

4) When asked to confirm storage device maintenance, choose Yes.

Now, some NAT problems are subtle and don't appear in the Test Xbox Live Connection test. Most likely there's a port blocked... ports can be blocked in your router, modem, or at your ISP. Since the Xbox 360 uses different ports than PCs & Macs use, you can't see the effect of a port blockage except on the Xbox.

A sequential power cycle is the first step in clearing a port blockage:

1) turn off the Xbox then

2) unplug the power from the router (Some folks have a combination modem-router AKA 'gateway'. If so, skip step 3).

3) unplug the power from the modem.

4) Wait at least 30 secs then replug the modem (or gateway) power.

5) Wait 'til the light on the front of the modem (or gateway) which says ONLINE, READY, CABLE or DSL is on solid (not flashing), then plug in the router. If you have a gateway instead of a modem and router, then when that light says it's ready, skip step 6.

6) Wait until the WiFi light on the router is on; you may not use WiFi, but it's still a good indicator your router is finished booting up.

7) Turn on the Xbox.  For more background on Ports, see whatismyipaddress.com/port

How does NAT affect Xbox LIVE gaming?

Symptoms:

When you try to join an online game or party, you receive an error.

You are frequently disconnected from a game or party.

You cannot hear or communicate with someone in an online game.

This sounds like You/Friend or BOTH could have Moderate or Strict NAT's. It is a very common problem. And will also affect your matchmaking or how  long it takes to find matches. Please Refer to this page for FULL Information on what the problem is and how to correct it.
http://support.microsoft.com/kb/979000 

Your NAT settings can cause this issue. I remember having it a lot when I was connected to my router. When you log into MW3, what does it say about your NAT setting. It is on the left of the screen underneath where you choose to find a match, start a private match or setup your classes.

Now the NAT settings are usually due to router settings. Specific router settings have to be changed in the router. To do this you must log into your router via the IP address. Typical IP addresses for routers are 192.168.1.1 but they can be different. You'll need to find the specific settings so that you can change your NAT configuration.

Thanks guys, I haven't fixed it I am kind of confused on how to fix the NAT settings. But on my xbox it does say my NAT is moderate.

Hey Nate, 

Thanks for bringing your issue here to the forums. Your moderate NAT could be causing the issue. You will also want your friend to make sure that his NAT is Open, as well. 

We have some simple steps you can use to correct the issue at our support page Error: Your NAT type is set to strict (or moderate)  Give those steps a try, and keep us posted on your results. 

Mine still will not let me join my friends party

My router/modem cannot be accessed. I have fioptics.

is there a way to join parties on xbox  for free instead of paying for it

no

if your xbox doesnt say nat then its coming from your "modem" or your isp open your modems settings and DMZ your router or set your modem to abridge mode or us ip passthrough form modem to router . call xbox if it continues.