At reboot, beastmasters will have access to the skill "mount lock". Locking a
mount is a toggle on/off switch. A locked mount cannot be ridden or led by
anyone it does not trust, that is, anyone who has not ridden it for an
extended time already.
To allow someone else to ride the mount, you will either need to unlock it and
let them ride it (then lock it again), or use the skill to manually add their
name to the list of trusted riders, which this skill can also do.