Minor characters, passers-by, crowds, and any other dialogue that does not belong to the principal characters is recorded during an Ambience session. Several actors are invited to these sessions, up to ten if the show requires a lot of different vocal colors, but at least two men and two women. Ambience sessions have to roll quickly, and a high degree of concentration will be required of the sound technician. This is also where less experienced actors cut their teeth - they will appreciate your patience.
Talking crowds, or walla, should ideally be recorded in stereo (recommended) although it is possible to cheat by recording a second pass of the same scene. Reusing of recorded materials is practical and will save you time and money. If the audio for the show has already been assessed and you find that you can get away with stock sounds, take that route. Some walla is show-specific, as for example when a dog, a lion and an alien enter a bar and the bar patrons reacts. There will likely not be a stock sound for this scenario, therefore specific walla should be recorded.