[Gmsh] How to group nodes

[saville at nodai.net]

Dear gmsh,

First let me introduce my self, my name is Ramadhona Saville, Tokyo University of Agriculture. I just started using gmsh today, and know nothing about that.

I would like to ask how to group some nodes as boundary condition, for example I made a simple 2D rectangular mesh (without using z coordinate), and I want to make the four sides as boundary condition for fluid flow simulation. 
Could you please show me how to do that? And how save that group, so that i can use its coordinate as boundary condition for fluid flow?
I have searched in the tutorial about that, but i just can not find about how to do that.

Thank you

Yours

Ramadhona Saville
TUA
International Bio-business Department
Network Information Lab