|The acquisition of new powers should be a fairly rare thing and fully justified in character. As rule of thumb, when originally generating a character, you can define a couple of new powers (known as latent powers) that you plan to acquire sometime later in the game. You can pick up the latent powers at any time with permission from the GM. Thereafter, new powers should be very rare.|
Players can spend their EXP on existing abilities, to make them more powerful. They can also spend EXP on stats as long as improving the stat makes sense for the character. Nominal human max for primary stats is 23 and you should have a good concept reason for increasing a stat above that.
You can also spend EXP to buy new abilities that are similar to abilities that you already have. For example, if you already have a teleportation multipower, then you can add a new slot to the MP that allows you to teleport further, or teleport more mass, or teleport other people, etc. In that case, you are taking a power that you already have and adding different advantages or adders. Even this, however, should be run by the GM first to make sure he has no issues with it.
You can also spend points on equipment that you find. If you use equipment that you have not spent points on, it will tend to be unreliable. For example, if you use a 90 AP gun that you haven't spent points on, it may misfire every other time you try to fire it.
Note that these rules apply specifically to the Gamma Company campaign. The Black Moon campaign is a bit more liberal (though new power acquisition is still controlled) while the World of Shadows campaign is even more liberal (where new powers can usually be justified by doing research to learn a new spell). Basically, in Gamma Company, I want the characters to develop much more slowly.