adverb
Used when two or more people do something together, often with a nuance of mutual participation or shared experience. Slightly formal or literary in tone.
家族共々に新年を祝った。
The whole family celebrated the New Year together.
夫婦共々に参列いたします。
My spouse and I will attend together.
共に is a broader adverb meaning 'together' or 'along with', while 共々に is a reduplicated form that adds emphasis on the togetherness of all parties involved.
Reduplication of 共 (とも, 'together') with the adverbial particle に. The repetition intensifies the sense of mutual togetherness.