Now that we can filter our friend list in various ways and can invite/block the players we have recently played with there should be more ways to actually get in party with people we have in our friend list.
As it is now, you can only see if someone is online or not. You can't see if they are in game, in lobby or in the main menu. So, if you don't have any external means of contacting with them and wanna invite them to party your best shot is inviting them every now and then and hoping that they eventually finish their game and get your invite.
Friends should have an additional status telling you whether they are in game, in lobby or in the main menu.
Aditionally, as suggested a milion times already (didn't count so not sure) a voice or text chat for your party would be absolutely wonderful.