When you join stitches "in the round" you create a continuous tube -- connecting the first cast on stitch to the last. Here's How to Join in the Round on 2 Circular Needles:

1. Distribute your stitches evenly around the needle
2. Hold the needle with the first stitch you cast on in your left hand and the needle with the last stitch you cast on in you right hand. Check yourself: Is your working yarn -- the yarn connected to the ball -- attached to the stitch on your right needle?
3. Ensure the cast on edges of all stitches are facing in the same direction. This prevents the stitches from "twisting".
4. Place a stitch marker on the right needle to indicate the beginning of a new round.
5. Knit the first stitch on the left needle, pulling the yarn tightly to avoid a hole.
6. Continue until all stitches have been knit and you're back to the stitch marker. This completes one round.

When joining stitches in the round it's tricky to create a seamless join even if you knit the first stitch tightly. Here's a trick we like and something you might want to try. Here's How.

Before placing your stitch marker and knitting the first stitch, swap the first stitches on the left and right needles as follows:
1. Slip the first stitch on the right needle to the left needle.
2. Slip the second stitch on the left needle up and over the first stitch and onto the right needle.
3. Place a stitch marker on the right needle to indicate the beginning of a new round.
4. Knit the first stitch on the left needle.
5. Continue until all stitches have been knit and you're back to the stitch marker. This completes one round.