The photos below show closeups of two entrelac joins done with the same needles and the same yarn by the same knitter (me).
The first photo shows one common way to join two pieces of knitting in entrelac. The beige piece was knitted first, and the green piece was joined, every two rows, by working a ssk, using the last stitch of a right-to-left green row and a free loop from held beige stitches. For a neater join, the first stitch of the return left-to-right row is slipped, with the yarn in back.
Notice that there is beige showing through the green stitches. This show-through, also called grinning, is pretty inevitable when you are forming a decrease (ssk) with two colors of yarn.
Notice also, the second column of green stitches (counting left-to-right) looks somewhat distorted. This is a consequence of the slipped stitches pulling at every other stitch in that column and making the left leg of that stitch smaller.
The next picture shows a neater join that I invented. Instead of working together the selvedge stitches of the green piece and held loops from the beige, I pulled a long green loop from each beige stitch and used the loop of yarn to work two rows of green, from left to right and then from right to left.
There is no show-through of the other color here. The first column of green stitches is slightly distorted. This is because the ratio of green rows to beige columns (stitches) is forced to be 2:1 instead of the more "natural" 3:2 or 4:3. This makes the green stitches look a little "squashed" since they "want" to be a little taller. It's not bad, though, and really, slipping stitches as in the first photo adds its own kind of distortion.
The second green stitch column looks a lot better, though. That's another benefit to this method.
Here's a rough diagram of the structure of the join: