zfs allow someuser create mydataset gives the specified user permission to create child datasets under the selected parent dataset. There is a caveat: creating a new dataset involves mounting it. That requires setting the FreeBSD vfs.usermount sysctl(8) to 1 to allow non-root users to mount a file system. There is another restriction aimed at preventing abuse: non-root users must own the mountpoint where the file system is to be mounted.

zfs allow someuser allow mydataset gives the specified user the ability to assign any permission they have on the target dataset, or its children, to other users. If a user has the snapshot permission and the allow permission, that user can then grant the snapshot permission to other users.