[hellmoss]

Creating a solid_bbox entity around the player with movetype = follow isn't the greatest trick because it blocks the player and if it's not solid it wont be able to take damage.. I saw a semiclip plugin in which players are inside one another but can still take damage from enemy fire (https://forums.alliedmods.net/showthread.php?t=137980) I'm being kinda lazy to look into it but from a quick look I don't see anything else manipulated but the solid state of players.

[Natsheh]

Have you tried to set entity ownership to the player that it follow?

[JocAnis]

Make that like you started, good idea.. Then : do a pev_groupinfo thing between only that player and his box
Meaning: set pev_groupinfo to player and reversed pev_groupinfo to his, followed box... So he wont ne able to touch nor see it, for others all normal