The most important thing when joining your artificial grass is to obviously hide it as best as possible. Here's how to guarantee you won't see the join.
Cut the underside of your grass as close to the stitch/seam as possible, being careful not to cut into the stitches otherwise you'll find the grass will fall out of there. This ensures the grass fibres are as close together as possible, if not you'll certainly notice a gap in your grass at the join.
Then, when taping it together (Bunnings has a great product) make sure you don't get any of the grass fibres caught to the tape. Now just apply plenty of pressure to ensure a clean, strong adhesion - I used my hands and feet but when I was at Bunnings yesterday I did notice Tuff Turf have brought out a new tool which you may want to try. Although it wouldn't be too hard to come up with something yourself.
The left side of the photo shows what you should do, the right shows what you should not do, ie. cut too far from the stitch. (Note: the top-right of the photo is fine). - ProjectPete