It depends what you mean by "join-up". If you are attempting to join a solo rider and are close to him/her (less than 2 seconds gap), I would say you are a little to close. And that maybe why you they are pulling away from you. You're just too close for comfort. I know if you did that to me, I would not want you that close.
I usually will hang back 3 or 4 seconds behind some one I don't know. Sure, having 2 bikes together is safer, if they are in a normal riding stagger with a normal interval. If you are 5 or 6 seconds back, it buys you nothing.
I look at a couple of thing when getting close to another rider. First, which side of the lane is the bike in (this depends on which lane they are in)? Left, center or middle? In the right hand lane, left side great. That means he/she is trying to protect their lane. Center, no way will I follow them. The center is when all the oils accumulate. Right side? I'm bailing out! The person has no clue on how to ride. Riding in the right side of the lane exposes the all of the lane to cars, trucks and whatever to pass or occupy the lane with you!
I will usually follow 3 to 4 seconds back from another rider I don't know and use a stagger. It's safer for me. If you are 3 or 4 seconds, in a stagger, behind as a rider that joined up with me, I'll keep and eye on you. But for me, that's a good interval. Even look like you don't know how to ride, and I'm gone.