If you're going into web dev, HTML is a must, for everyone on your team. CSS will help a lot for everyone too. Javascript for client side and PHP for server side you can split up on, maybe have half the team learn each.
C and C++ is not a big language used for web dev. You can use them, and a lot of people did/do with cgi scripts, but C++ and web dev don't really go together. C++ experience will be very valuable in helping them learn PHP and javascript though.
The forums will be in PHP and MySQL most likely. What type of games you want to make will be important. If there is tons of server side communication, like games that you don't start over fresh each time, then you'll probably use PHP or java. If you just have something like pinball, then flash would be good. These are just the general tendencies though, and you can go against them no problem.
(keep in mind that these are just my opinions and I am by no means an expert)
good luck!