According to the Prima Guide there are certain days and times when the places are 'rocking'. I had my Sim go at these times and there were more than the 1 or 2 'customers' that normally showed up.
* Dive Bars are busiest at 2pm for the lunch crowd on Wednesdays and Thursdays. Maximun guests are 8.
* Dance Clubs are busiest on Saturdays at midnight. Maximun guests are 14.
* Lounges are busiest at 11pm on Fridays and Saturdays. Maximun guests are 10.
I tried this at both Bridgeport and Twinbrook. I didn't try SV or RV as I haven't played those towns in awhile. Hope this helps.