Once you join the frontline group you will need to join a beta game in the first week, and will need to remain active in beta games without more than a 4 week gap between beta games.
On the game list you will see the beta games that are system generated will start the title with [BETA] and also have a tag across the corner listing that. You may have to scroll down in the new game list in order to find them.
To separate these from standard games, there is a search feature as well. A shortcut to the search for beta games is on the bottom of the Frontline Pioneer tab on the main page. If there is not one currently open in one language you can always switch languages and see if there is one available there.
There should be Frontline games available in both English and Portugese servers. I am sure there is always one available on the EN server, I am not sure how frequently they are generated on the PT server.
The offer to be a Frontline pioneer is given automatically to players that qualify when there are openings. I am sure that if you have been one before that you will get the offer again.
