For the duration of the eight days of Chanukah (Dedication), we recite Hallel (Psalms of praise) with a Beracha (blessing) before and after—the full Hallel. It is customary to stand during Hallel, not to sit. But if a person is ill, infirm, or unable to stand, they can say Hallel while sitting.
Saying Hallel with a minyan (quorum of ten) and singing together certainly enhances it, but saying Hallel is not limited to a minyan. If a person is praying alone, they would still say Hallel with a Beracha before and after.
A person has the entire day to say Hallel. Although we typically say it as part of Shacharit (morning service), if a person for whatever reason missed Shacharit and remembered in the afternoon that they had not said Hallel, they can say Hallel independently with a Beracha before and after, up until sunset. If they remember after sunset but before nightfall, they can say Hallel without a Beracha during Bein Hashemashot (twilight).