comitor is deponent in the middle voice which is somewhat like a reflexive in spanish .... as I understand it.
From greenough #397, under iomatic uses of the accusative,
In many apparently similar expressions the accusative may be regarded as the direct object of a verb in the "middle voice"
inutil ferrum cingitur he girds on the use
In section 156 it is explained that the middle voice has reflexive force which is confirmed by GLOSSA's entry for comitor:
I. v. dep. a. [comes], to join one ' s self to any one as an attendant, to accompany, attend, follow
and another example from GLOSSA
propinqui Indutiomari comitati eos ex civitate excesserunt
Attending them, indutiomarus' relatives departed the realm