Evanescing is the formal term for what is usually (and near-exclusively) referred to as teleportation or linking in online circles, the act of abruptly moving from one place to another amongst online landscapes, available to both Citizens and Denizens.
The word was initially chosen in homage to early internet habits of prepending e to concepts relating to the web (such as e-mail) and to distinguish it from ongoing attempts in physics to get actual, physical teleportation to work.
Self-initiated teleportation is limited only by the access control of the intended destination. Sanctuary, like all servers and networks, does not impose exit rules and cannot contain a Denizen against their will.
So-called symbiotic teleportation (or just Following) involves a mutual agreement between Denizens to stay together on teleportation terms: If one user teleports from one area to another, others part of the agreement will follow. If users end up out of each other's visual field and earshot, servers commonly send a reminder to them about the bond, but do not sever the tie, themselves. (Sanctuary is no different here.)
To establish symbiotic teleportation, two parties must initially actively request the bond, and any further additions to the web require one participant in the net to acknowledge the request of the person in question. To terminate symbiotic teleportation, only the party wishing to withdraw must act. There is no way of expelling someone from a group (it was never implemented into the protocol because it's such a rare occurrence) - disconnecting from it and forming a new set of connections would be a way to get around this restriction, though.
Commonly, symbiotic teleportation agreements involve only two people, but in theory there is no limit to the amount of people that can connect this way.
Administrators (or other users with RELOCATE access) can remotely teleport people from one place in their Fragment(/server/network) to another place in their Fragment(/server/network). This is distinct from kicking or banning actions and only works within the sphere of the admin's control.
Since it's possible to reassign actions to access levels arbitrarily and with quite fine-grained control, in theory, it's possible to protect a user from this interference by instructing the Fragment(/server/network) to require higher access to displace the user in question.