Select the vertices you want to be in the vertex group.Use any method you want.
Go to Mesh->Vertex Groups and click the + sign to add a new group. Type in a name for the group.
Click Assign. Later on, if you want to select this group of vertices again, come back to Mesh->Vertex Groups, click on the group, and click Select. The vertices in the group will be selected along with whatever you already had selected. Click Deselect to unselect the vertices in the group.