Guest Addressing for Invites

Are you having inner and outer envelopes? If so, you can put their full name on the outer envelope and then their nickname (or whatever you normally call them) on the inner envelope.

That’s what outer and inner envelopes are for. The outer is for formal names/titles and the inner envelope is for names they go by.

The formal way to include a nickname would be Mr. William "Bill" Doe. If you definitely want to use the nickname rather than full name, I would personally still include the Mr/Ms.
We so did black tie optional and addressed things formally, except for a few non-binary friends who told us they'd prefer no title, so we just used perfered name, but it did look funny without a title next to all the others that did have one.

View Quoted Comment I recently learned that for non-binary or agender people, you can use "Mx."

View Quoted Comment Yep! But I learned none of my people like or use that. Apparently many people in the community don't like it, so better to ask them directly for their preference!

We're doing formal titles with full names on the envelopes, and then nicknames on the seating chart/place cards. So for instance, my father in law to be is William, but goes by Bill. So his invite is Mr. and Mrs. William Doe but his seat will say Bill Doe.

Now if you are handing out the invitations then using their nick names are ok. But if you are mailing them out you have to use there proper name if you are sending to their homes.