It depends on the popularity of the band in some cases.
Bands like Take That will do a formal 'Meet & Greet' which will be arrange for comp. winners and be strictly timed and supervised in advance.
Most bands who are simply touring don;t do casual backstage access for safety and security reasons. Backstage can be quite dangerous, and crew don;t like strangers wandering about getting in the way.
As a writer, i often get access, but it's to do an interview, and when it's over, out i gto, i don;t get to 'hang out' with the band - it's just not the way it works.
Afterwards, selected family, friends, guests and so on, can come back for a drink and nibbles if the band are not straight off to the next venue, and they often are, but again, that's done by invitation only.