VirtualBox shared folders are owned by root in Lubuntu guest

On Ubuntu Server host execute these commands :

sudo chmod -R 777 /path-to-shared-folder/shared-folder

sudo chown -R user1:user1 /path-to-shared-folder/shared-folder  

On Lubuntu Desktop guest execute this command :

sudo usermod -G vboxsf -a user2

Restart the guest system for changes taking effect.

Note : user1 = your host user name | user2 = your guest user name

No need to change main group of user - add user into group is sufficient:

sudo adduser myuser vboxsf

It is not necessary to change the permissions on the host system, just easily mount the shared folder for the normal user:

sudo mount -t vboxsf folder share -o uid=1000,gid=1000

1000 is the default ID of the default user. This can be checked by id username