There are a ton of vehicles we can glue on without VIP, which is great.
However it would make the most sense to have pick-up trucks added into the list of vehicles we can glue on for free. People ride in the backs of pick-up trucks in real life all the time:
(https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRlr88vg8sMgZebPTRgskvoX3J_5RV5FMAB9me63jyZ_nh02bvK)
It's realistic and pretty cool. I'd say it's best for friends who just like to roll out together, or even kill cops with style.
(http://i.imgur.com/wyiWB2q.png)
These are all the pick-up trucks we have in car shops, and should all have the feature of free gluing.
Bobcat
Show content
(http://www.beamng.com/attachment.php?attachmentid=30832&d=1402099143)
Walton
Show content
(http://img1.wikia.nocookie.net/__cb20100513220927/es.gta/images/2/2e/Walton_SA.jpg)
Sadler
Show content
(http://img4.wikia.nocookie.net/__cb20090921091828/gtawiki/images/a/af/Sadler-GTASA-front.jpg)
Yosemite
Show content
(http://img2.wikia.nocookie.net/__cb20091105085052/gtawiki/images/1/17/Yosemite-GTASA-front.jpg)
Picador
Show content
(http://img1.wikia.nocookie.net/__cb20091105082009/gtawiki/images/d/d1/Picador-GTASA-front.jpg)
Now, there's not much space in the rears of these trucks for loads of people to be glued on (and have enough space to be fully seen and shoot without the blocking animation). So with that said, there can only be a max of 2-3 people glued on the back of a truck (maybe that could depend on how much space is in each separate type of truck).
Other than that, everything about gluing onto other vehicles applies to these as well.