When attempting to teleport a mob to a player I get 'The entity UUID provided is in an invalid format'

Solution 1:

With this command, you are trying to teleport all entities called Steve to all members of the team Disgiused.

This is impossible. You need to define an unique target, not a group. Use @r (Random) or @p (nearest) instead.

Solution 2:

With @a[team=disciused], you are trying to teleport the entity to all the players in the team, which is, in short, impossible. To teleport this person to a random player that is in the team, you can use this command:
/tp @e[name=steve] @r[team=disciused]

Or to teleport them to a specific player (or entity) in the team:
/tp @e[name=steve] @p[team=disciused, name=playername]