In the case of non-booked visitors/regulars, most clubs I have been involved with have been able to organise some sort of assistance to singers who wished to work on their singing, and most of them have met with some degree of success.
I have never been convinced about the one-to-one "teaching" of people to sing, but the most successful I've known were those where a group of singers/enthusiasts have met regularly on a formal basis, asked 'wannabes' to bring along a number of songs to be worked on, then worked on them in a chaired discussion around the strengths and weaknesses of the performance, finally making specific suggestions of how to approve the singing – positive and negative suggestions being the rule of the day.
We always avoided telling people "how to sing", but if we were going to put in time and effort to help we did expect the singer/s to show that they had put in the work between meetings.
The democratic nature of this set-up was a two-way street; hopefully the singer/s took away something to work on and the rest, by being put in a situation of having to think about what was happening, picked up tips that they could apply to their own singing.
A bit of a simplistic explanation of the work; some of these 'workshops' took the work much further than ths explanation.
Jim Carroll