General Information:

Here the words "we," "us," and "our" refer to John and those who had been with Jesus. (See: rc://en/ta/man/translate/figs-exclusive)

That which we have seen and heard we declare also to you

"We say to you also what we have seen and heard"

"be our close friends. We are friends with God the Father"

It is not clear if John is including or excluding his readers. You may translate this either way.

These are important titles that describe the relationship between God and Jesus. (See: rc://en/ta/man/translate/guidelines-sonofgodprinciples)

"to make our joy complete" or "to make ourselves completely happy"