When a compound subject involves two or more people, "their," "they" and "our" are preferable to using singular pronouns to represent each person. For example, "Charles and Ann are taking a walk" sounds awkward written as "He and she are taking a walk." "They are taking a walk" is the more logical choice. In the same example, if a preceding sentence introduces one person, you can name the new person and use a pronoun. "He and Ann are taking a walk."
Using the possessive compound pronouns "their" and "our" can add interest to writing and avoid repetition. "Michael and Jonathan are waiting for Michael and Jonathan's mother" is better written as "Michael and Jonathan are waiting for their mother."
The Elements of Style by William Strunk Jr. and E. B. White urges writers to reconstruct a sentence when the antecedent is indefinite. Using a plural antecedent or eliminating the pronoun are "strategies to avoid an awkward overuse of 'he' or 'she' or an unintentional emphasis on the masculine." For example, "A baker must consider his customers' food allergies," can be rewritten as: Bakers must consider their customers' food allergies.
or: The baker must consider customers' food allergies.
Whether a noun is common or possessive, generally the rules of using a pronoun in its place remains the same. With common nouns, remember there is a distinction between plural and compound. In the example, "The balls are in the locker," the common noun "balls" is plural because there is more than one ball. The noun is compound if there is more than one subject, as in the example, "The bats and balls are in the locker."
Both the plural and compound subject are replaced by the same pronoun "they". Because they use the same pronoun you may need to understand the sentence in context to determine if the subject or object is plural or compound.
You may have been taught to use "Sara and I" rather than "me and Sara." This lesson in etiquette urges you to consider the other person before yourself; it does not mean to use the "Sara and I" construction in all situations.
Remove the second personal pronoun from a sentence to determine whether "I" or "me" should be used. For example, "There are enough cookies for Sara and I to share" becomes "There are enough cookies for I to share." Using a singular subject shows you that "I" should be changed to "me".