A guest singer is booked and usually paid to sing. The guest will usually sing for most of the evening.
A floor singer turns up unannounced hoping to sing a couple of songs, and does so if he/she gets lucky and is invited to do so. He/she is not paid. Most clubs have guest nights and singers nights. On a singers night, all the music is made by those who turn up on the night. On a guest night the guest is the main attraction, but the club may ask a singer from the floor to do a couple of songs at the start, for example, as a warmup.
"Residents" are people who attend frequently and will sing if asked. The host knows they will be there, and knows they will sing if asked.