I have only one friend on my friends list that when either of try to join or accept a fireteam invite we get "Unable to join target player. Please try again". I can join any of my other clan members or friends without any issues. We're not on the same network and have played together before with no issues.
Hi there. Can you other friend play with clan mambers and their friends? It's possible that this is a weird NAT type issue. You may want to review our [url=http://www.bungie.net/en/Help/Troubleshoot?oid=13610]Advanced Steps Networking Guide[/url] for information on NAT types.