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.

Looks like they have since fixed it so you now can join ingame by right clicking on their name

Yep and for me it was only with the squad leader when 2 or3 were already in the party.