ihr
you
Second-person plural personal pronoun in the nominative. It is the informal form of address for a group of people.
Was macht ihr heute Abend?
What are you doing this evening?

you / her / their
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you
Second-person plural personal pronoun in the nominative. It is the informal form of address for a group of people.
Was macht ihr heute Abend?
What are you doing this evening?

You can recognize the function of 'ihr' (you/her/their) from the sentence position and the verb. As a form of address (subject), the verb ends in '-t' (ihr kommt). As a dative object, it comes after the verb (ich helfe ihr). As a possessive article, it stands before a noun (ihr Buch).
Ihr sprecht sehr gut Deutsch!
You speak German very well!
'Ihr' (you) is the plural form of 'du' (you). You use it to address a group of friends, family members, or children directly. The verb in the sentence is conjugated accordingly and ends in '-t'.
Hallo zusammen, was wollt ihr trinken?
Hi everyone, what do you want to drink?
'Ihr' (her) is the dative form of the personal pronoun 'sie' (she; feminine, singular). It is used as an indirect object with verbs like 'geben' (to give), 'helfen' (to help), 'antworten' (to answer), 'danken' (to thank), or 'gefallen' (to please/be liked).
Er schenkt ihr zum Geburtstag Blumen.
He gives her flowers for her birthday.
As a possessive article, 'ihr' can mean that something belongs to a woman ('her') or that something belongs to a group of people ('their'). Only the context makes clear which meaning is intended.
Maria telefoniert mit ihrer Mutter.
Maria is talking on the phone with her mother.
to be
'sein' and 'ihr' are both very common small words, but they have different functions. 'sein' is a verb and says that someone or something exists, has a quality, or is in a state. It can also be used as an auxiliary verb, for example in the Perfekt. 'ihr' is a pronoun or a possessive article: it can address a group directly, refer something to a female person, or express possession. Learners often confuse words like these because they are short and have many forms. The important point is: 'sein' is a verb in the sentence; 'ihr' refers to people or possession.

Nach der Arbeit sucht die Chefin ihr Handy im Auto.
After work, the boss looks for her phone in the car.
Die Lehrerin ist heute in der Pause sehr geduldig.
The teacher is very patient during the break today.