If you look up the radio term “Combine” this will answer your question.
You must agree all the points before takeoff, as has been mentioned. If the faster aircraft takes off second he can usually join without ever loosing visual contact with the lead aircraft.
Flying 1/3 mile trail is not the same as a close formation. Two experienced pilots flying appropriate aircraft can fly the length of the UK with little problem provided the weather is good.
If you are tail end charley you are checking the leads nav, and maybe doing the radio. The aircraft not doing the radio will be asked by ATC to squawk standby. In good weather, it can be very relaxing. The gotch is that if you stop paying attention to your position and you suddenly get separated you are on your own with a very big circle of uncertainty!
I flew to Wick for the PFA rally this year. I flew in combine with an RV4 on the way up, and on my own on the way back. It was much harder work on the way back, but it was all really good fun.
Rod1