How do you get more bitizens in tiny tower?

Provided you have residential space available you can either wait until a bitizen requests to be taken to the residential floor (at which time they will live at Tiny Tower) or you can select the residential floor you wish to fill and pay one Bit Bux to rent space immediately. If you have filled your residential space, you'll have to build more by clicking the "add floor" billboard at the top of your building, then select the "residential" type (the first option, at the top) and begin building a new apartment.