Sennheiser EM500 Instruction Manual page 49

Synchronizing transmitters with receivers
48
On one of the receivers, select a channel within this frequency bank
(see page 39).
Assign this channel one of the receiving frequencies (see page 39).
Synchronize a transmitter with the receiver (see page 22).
OR
Manually set the transmitter to the same frequency bank, channel and
frequency that you set on the receiver.
Repeat for the remaining transmitters and receivers as described
above.