Ok so as far as I know, you can only invite people to your party in-game. How you join a group is going to Origin and opening your friends list. You should see a blue "join game" icon that will let you join a group. You cannot add friends in-game. That has to be done through Origin. The groups are limited to 4 as usual with Battlefield games. Since you can't search and join a server list in the beta, you won't be able to get two groups together to play unless you randomly join the same game.