There is little 600 MHz spectral band available. Available unlicensed bandwidth is 614-616 & 657-663 MHz, A Part 74 license gives you access to an additional 4 MHz, 653-657 MHz. The rest 600 MHz spectral band is owned by the winners of the recent FCC auction for use as Band 71 for cell phone data.
This bandwidth is also limited to 20mW unlicensed or licensed so it would not be the best choice for this application.
As far as eligibility for a part 74 license, the requirements for a professional sound company as defined in the CFR (Code of Federal Regulations) "Routinely using 50 or more low power auxiliary station devices means that the professional sound company uses 50 or more such devices for most events or productions."
A Silent Disco only needs one frequency. This does not meet the requirement of a Part 74 license.
You can transmit without a license 470MHz-608MHz at 50mw without a license.