Is there a halacha that says if one is invited to a bris he must attend?
The Rema (Yoreh De’ah 265) writes that somebody who refrains from attending the se’udah (festive meal) of a bris is considered “excommunicated from Heaven.”
For this reason, the Pischei Teshuva (265:18) cites that one should not invite others explicitly to the se’udah.
However, somebody who can’t attend for reasons of being occupied — at work, or for another pressing cause — does not have to be concerned for this.
See also Otzar Ha-Bris no. 163.