A while ago I'd written about C's* Upanayanam ceremony. Not sure if it's the words in the comments section, but quite a few people come to the blog looking for Upanayanam invitation samples. I dug up the one we had used. Here's a photograph of it.
I fell in love with the image on the cover the minute I saw it. I thought it looked especially good in gold layered over an off-white background. We used the same motif on small red bags for the "thindis" (snacks) that you give guests before they leave and on large bags for gifts of saris, dhotis, etc. for the elders.
On the inside, there's text on both sides. The left side has logistical information: Date, the time for the "Akshata" (or muhurtham), Lagna, Venue and RSVP information. The right side has an image of the family deity and the text of the formal invitation: the names of both sets of grandparents "seek your presence at and blessings for the Upanayanam of their grandson [name] (son of [parents' names]). At the bottom, it says "With the best compliments of" and we listed the name of every single one of our siblings, sibling spouse and child.
* Then called N. See here for reason for name change.