Usually when building a character (or anything that has some symmetry), you work on one half with the other half mirroring it. For example, you create the right side of the body and mirror it to the left side. After you're done with all the details that need symmetry, you can keep both halves and then modify it with non-symmetrical details if it's required.
Assuming you're using the latest dev release, there's a tool in the Build->Mirrors menu. If you align the mesh to the right-side of the X axis and go to Build->Mirrors->Convert to Mirrored, and it'll mirror and merge a left-side copy to the right-side. It's an "active" object, so any changes you make to the right-side will automatically update on the left side.
Otherwise you can go to Build->Mirror Image and mirror across the axis and then manually merge the meshes and points.
Back to your original question, join the extremities to the torso by selecting both meshes and going to Build->Join Solids. If they are subdivision meshes then first you have to double click them and set subdivisions to zero, then convert to mesh, then Join Solids. It's easiest to use the
Topographical knife (k) tool to connect the two together, but you can also use the Add Edge tool. Both of these tools are in the left toolbar while in Point-Edit mode.
An easy way to join, say, an arm to the shoulder is by cutting a large face at the shoulder socket with the same number of sides as a large face on the shoulder-side of the arm. Then select both faces in point-edit mode and go to Edit->Bridge (=)
