Learn Spanish (Latin America): Mis amigos y yo inventábamos juegos en nuestra infancia. - My friends and I used to invent games in our childhood.

There isn't a separate word for it! The "used to" meaning is built right into the verb. In Spanish, the imperfect tense (like the -ábamos ending here) is used to talk about past habits or things you used to do repeatedly over a period of time.

Why is the verb inventábamos in the "we" (nosotros) form?

The subject is Mis amigos y yo (My friends and I). Whenever you include yourself in a group with y yo (and I), it is the equivalent of "we", so you must use the nosotros verb ending.

Why does inventábamos have an accent mark over the first "a"?

For regular -ar verbs in the imperfect tense, only the nosotros form gets an accent mark. The ending is always -ábamos. The written accent ensures your voice stresses the correct syllable (in-ven-TA-ba-mos), matching the rhythm of the other forms.

Why do we use nuestra instead of nuestro for "our"?

Possessive words like "our" have to match the noun they describe in both gender and number. Since infancia (childhood) is a feminine, singular word, you must use the feminine, singular form nuestra.