To play Mugen Online You need Evolve installed, A copy of Ikemen (both users need the same characters otherwise you get blanks in your roster)

First Step is to create an Evolve Account, after creating your Evolve Account, add your friends/opponents by heading over to upper left "Evolve" Tab an selecting the button "Friends" . After clicking, a small window will pop-up revealing your list of friends. Click the RED Friends button and add whoever you wish by entering their ID.

 

Second Step is to create a PARTY, click on the upper left Evolve tab and click on

CREATE PARTY and invite your FRIENDS, once their in the party, RIGHT CLICK on their IDs and click on "COPY IP ADDRESS"

 

Third step is to load up ikemen (Located in your Online Mode folder within RBX 2) and select NETPLAY, an Input server window will appear, If YOU are HOSTING a match press OK.
 

If you are CHALLENGING someone you will enter THEIR IP into the Input Server window, and It will almost instantly go to the select screen.