Ok, I'm here you answer your question:
First, you have to add some friends.
There were two ways of adding friends:
1. From the Ranking List.
This was simple, but it has restirictions. Go to the Ranknig List, pick a player you want to add from the list and click on "add friend" button. It you could add them only if you could see them. This feature has been removed in one of the latest patches(i don't know which one)
2. Using the Friend ID.
This is the only way to add a friend currently. For this, You'll need the Player ID of the player you want to add. Unfortunately, there are no ways to get this in-game, you have to know someone playing supermechs and request the Player ID from him/her or you must find people to add as friends through the forum, but since the Player ID's are actually the User ID's of this forum, you can use them to add other people too. Here
is the link of the topic on how to do that, which is written by myself
Second, you have to invite yopur firend to battle. To do that, go to the friends section, it will show all your friends added in supermechs. The green backgrounded ones are the online friends, while the black backgrounded ones are the offline friends. The single yellow backgrounded friend is the friend you have clicked on and it will not change color regeardless of your friends is online or not, you have to actually click on somebody else to see if (s)he is online.
Select a friend who is online, and click "invite to battle" button. The friend will get a blinking crossed swords sign in the place of the friends section button in the main interface, instead of two people sign representing "friends". You'll have to wait until your friend accepts or rejects your invitation, or you can cancel the invitation. While you wait, your friend must go into the friends section, click on your name and choose one of the buttons to accept or reject the battle invitation. IF your friend accepts, the typical vs screen of multiplayer battles will appear and you and your friend will than start to battle, good luck!
I hope I was able to help. Cheers