You are permitted to formate on a squawking aircraft, and there is no published limit to the size of the formation.
Impromptu formations without a thorough briefing beforehand and a clear definition of the roles and responsibilities of all participating pilots are not legal.
You can't just formate on someone and ask if you can tag along! Even on the ground, if someone asked me before takeoff to lead them through the zone, I wouldnt't do it if I wasn't sure of their skills and trustworthiness.