Learn Spanish (Latin America): Caminamos juntos a la reunión.

No, that would sound like "we walk with ourselves." To say people are doing an activity together, Spanish uses the word juntos. If you want to use con, you need to specify who you are walking with, like camino contigo (I walk with you) or caminas conmigo (you walk with me).

Why is the word for "we" (nosotros) missing from this sentence?

In Spanish, the ending of the verb tells you who is doing the action. The -amos ending on caminamos already means "we walk," so you do not need to include nosotros unless you want to add extra emphasis.

If a group of women is walking to the meeting, do they still use juntos?

No, they would say juntas. Unlike the English word "together," the Spanish word junto acts like an adjective and changes to match the gender of the people it describes. Use juntos for a group of men or a mixed-gender group, and juntas for a group of only women.

Does caminamos mean "we walk" or "we walked"?

It can actually mean both! For regular verbs that end in -ar like caminar, the "we" form is exactly the same in the present tense (we walk) and the past tense (we walked). You just have to figure out which one it is based on the context of the conversation.