Once you have created all of your lofts you should attach each surface together. First, select the the first surface, then (holding down shift) select the second surface. Then, using the attach surfaces command (go to edit nurbs> attach surface) attach them together. Repeat this step until you have a single solid nurbs surface.
Now all we have left to do is match the nurbs surface to the inside of the mouth. In order to do this we will need to match the information up. Ignoring the center edge and the mouth corner edge... count the number of edges that make up your upper lip. In this case we have edges.
In order to match up the edges with the inside of the mouth, you must insert 8 isoparms from the center edge to the mouth corner edge. In order to do this, right click over the nurbs surface and select "Isoparm" from the marking menu.
Select an edge and drag. You will see a yellow line appear, this is not actually an isoparm... it's just a marker indicating where you want to put an isoparm. Holding down the shift button you can insert as many yellow lines as you need.
After you have created 8 evenly spaced yellow lines, you need to create isoparms out of them. To do this, simply go to edit nurbs>insert isoparms and the information will be added for you.
When you are done repeat this step for the lower lip as well.
Finally we have a surface for the inside of the mouth. The next thing we need to do is convert it to polygons so that we can attach it to the rest of the head. To do this select your nurbs surface, then go to modify>convert>nurbs to polygons and open the options dialog box.
Under "Type" select quads
Under "Tesselation Method" select general
In the Initial Tesselation Controls section...
for "U Type" select "Per Span # of Iso Params"
for "Number U" select "1"
for "V Type" select "Per Span # of Iso Params"
for "Number V" select "1"
Then press the tesselate button in the lower right to convert it to polys.
Now that you have a polygon version of your mouth you can delete the other nurbs components used to make this surface.
Before you can connect the two shapes into a single mesh you should snap the verticies of the inner mouth shape to those of the mouth on the head. Go into component mode and select one of the verts of the inner mouth edge. Holding down V for vertex snap move it to the corresponding vert on the head shape. Repeat this step untill both edges are perfectly lined up.
Now that you have both edges lined, up the first step is to combine your geometry. Select both pieces then go to polygon>combine to join them. This will make Maya treat your 2 pieces of geomety as one piece. However, there still is a border edge seperating the 2 pieces. This will be evident when you go to smooth your mesh. If there is a border edge a huge seam will appear.
To view your border edge, go to display>custom polygon display and open the options dialog box. In the "Highlight" section, check border edges and in the "Border Width" section, put the slider to 5. Click apply and close. This will draw your border edge a little thicker than the other edges to help you see what your doing when you start to merge edges.
The next thing you're going to want to do is merge your border edges together. To do this, select your mesh and go to edit polygons>merge multiple edges and open the options dialog box.. Set your threshold to .01. This is important because if you set this number too high, it can merge your mouth shut. Press apply to merge edges. If there are some edges that didn't merge try setting it to .02 and go up from there until all your mouth edges are merged.
If your mouth didn't merge at all... you might have your normals flipped. If this is the case, go into component mode and select one face from either the head or the inside of the mouth, then go to edit polygons>normals>reverse and open the options dialog box. Set the mode to "reverse and propogate", then hit apply. This will flip the normals on the offending section, then you should be able to merge your seams.
When you are done you should have a single piece of geometry with no seam.