To select a block, simply click on it. Blocks of text are completely independent - you can move them around by clicking on them with the left mouse button and dragging, you can delete them front the "Edit" menu, and you can change their colour and font without affecting other blocks.
Different users can work in different blocks simultaneously without affecting each other. In fact different users can modify the same block at the same time, except that a block only has one colour, font, and location, so if two users attempt to modify these block attributes simultaneously, one of them will win. Similarly, if two users attempt to modify the same line simultaneously, one of them will win, although the pen icons should enable users to figure out what it was that happenned.