See man vfs, search vfs.usermount, hence merely enable user mount by run sysctl vfs.usermount=1
See man devfsctl, investigate the section EXAMPLES, add some lines into /etc/devfs.conf, in my case, e.g.:
perm da8s* whom:whom 0644
service devfs restart and plug into an USB stick with, say, msdos filesystem, the device is created as /dev/da8* names in my system:
root@df:~ # ls -l /dev/da8*
crw-r--r-- 1 whom whom 33, 0x1e110047 Feb 28 10:32 /dev/da8
crw-r--r-- 1 whom whom 33, 0x1e120047 Feb 28 10:32 /dev/da8s1
crw-r--r-- 1 whom whom 33, 0x1e130047 Feb 28 10:32 /dev/da8s2
crw-r--r-- 1 whom whom 33, 0x1e140047 Feb 28 10:32 /dev/da8s3These dev files should be able to mount properly under some directories owned by whom.