Here is the basic wording for an invitation.
Change the beginning lines to personalize it. (see below)
Print this out for a reference, or cut and paste into my inquiry / order form and then modify it with your details:

With pride and pleasure
we invite you to worship with us
when our son
Jonathan Edward
is called to the Torah as a Bar Mitzvah
Saturday, the second of August (Saturday, August 2, 2007)
Two thousand and seven
at ten o'clock in the morning
Temple Rodef Shalom
Pittsburgh, Pennsylvania

Ellen and Scott Stein

or...is called to read from the Torah
on the occasion of her Bat Mitzvah

or...becomes a Bar Mitzvah

Here are some alternative opening lines for the above wording:

In honor of a beautiful tradition
our daughter...

Share our happiness...

Please share our simcha...

Join us in celebration
when our son ________
is called to the Torah and becomes
an adult member of his community  

We warmly invite you to join us...

Celebrate with us when our daughter...

It would be our pleasure
to have you worship with us...

With joy we invite you
to share a special moment
when our son...

Please join us as we celebrate...

In the tradition of his ancestors, our son _____...

This wording can be used for different family situations
Here the child issues the invitation:

My family and I would be honored
to have you join us
on the occasion of my Bat Mitzvah

..

Please join my family and me
as I accept the responsibility
of my Jewish heritage
by becoming a Bat Mitzvah
...day, time, place

Jennifer Alice Schwartz
.....