Heorðgenéatas and þegns
The actual Old English word repeatedly used in Beowulf is heorðgenéatas, which translates literally to 'hearth-companions', or alternatively gesíð (gesith) or gesteallum, which also translate as 'companion'.
þegn (thegn, or thane) can mean 'retainer' or 'servant', but came more generally to mean a minor member of the landed aristocracy, as described in Geþyncðo (Dignities):
"And if a ceorl throve, so that he had fully five hides of his own land, church and kitchen, bellhouse and burh-gate-seat, and special duty in the king's hail, then was he thenceforth of thegn-right worthy."
Thus, every heorðgenéat was a þegn, but not every þegn was a heorðgenéat.
There are two problems with using the term comitatus to describe a group of Scandinavian warriors (despite being a famous English work, Beowulf is set in Denmark):
firstly, it's a Latin term and thus definitely not what the people themselves would have said, and secondly it's used at least 400 years prior to the writing of Beowulf, and to describe a different culture. This is about as accurate as describing them as knights.
A related term which comes into use late in the Anglo Saxon period to describe an elite group of warriors surrounding a king or lord is huscarl - these were not thegns as they held no land, and were instead paid in coin.