@SubscribeEvent
  public void renderTeamBar(RenderGameOverlayEvent event)
  {
	  if(event.isCancelable() || event.type != ElementType.EXPERIENCE)
	  {      
	      return;
	  }

          String  team = this.mc.thePlayer.getTeam().getRegisteredName();
	  
	  if(RoundHandler.isRoundaktiv) //that is a static boolean
	  {
		  FontRenderer fontrenderer = mc.fontRenderer;
		  mc.getTextureManager().bindTexture(new ResourceLocation("minecrafttrouble:textures/gui/teambar.png"));
		  
		  GL11.glColor4f(1.0F, 1.0F, 1.0F, 1.0F)
		  GL11.glEnable(GL11.GL_BLEND);
		  OpenGlHelper.glBlendFunc(770, 771, 1, 0);
		  GL11.glBlendFunc(GL11.GL_SRC_ALPHA, GL11.GL_ONE_MINUS_SRC_ALPHA);
		  this.drawTexturedModalRect(this.xPosition, this.yPosition, 0, 0, 100, 20);
		  this.drawCenteredString(fontrenderer, team, this.xPosition + 100 / 2, this.yPosition + (20 -  / 2, 14737632);
	  }
  }

 

The Problem is:

 

In a Lan-World only the Host see the Gui, noone else.

How I can fix that?

In Minecraft there are networking packets that go between the client (GUI side) and server.  This actually happens even in single-player mode.

 

For the built-in Minecraft and Forge there are several packet systems already in place.  TileEntity uses packets to sync information, the data watcher uses packets, containers use packets, chat uses packets, commands use packets, etc.

 

The problem is that if you're not using one of those then you have to send your own custom packets.  The currently recommended way to do this is to use the simple network wrapper that has recently been fully implemented in Forge.  You should use diesieben07's tutorial to set this up in your mod: http://www.minecraftforge.net/forum/index.php/topic,20135.0.html

 

It is a bit of work, but it is worth doing.  You'll find many times in modding when you need to sync client and server with custom packets.