If you come by metro or on feet, as you go out of Colosseo metro stop (B line), cross the street and keeping the colosseum to the left, you'll get to the arch, the meeting point is to the right of it, at a small column close to a street lamp. Search for Rome in tours sign!
The tour ends at the roman forum, very close to the church of Saint Peter in chains, where you can find the Moses of Michelangelo, and you'll be close to the Metro stop Cavour B line, 1 stop from Termini station