I have been asked to inform a certain employee that he needs to attend an informal meeting that will tackle his capability. I haven't done this before, so any suggestions on how to write the letter?

Why need to write a letter when it is informal? You can just pick up the phone and ask him to join you.

I agree. You can tell him verbally about the meeting regarding his capability. And if you come up with concrete conclusions on the meeting, you can confirm these in writing to him.

Or you can simply write that you are inviting him to join an informal consultation meeting and that the meeting is intended to be an honest and open discussion.

If this is really a meeting that you think could possibly result in disciplinary action, then yes, it is best that a letter is written. And if you yourself think that this meeting is more formal than what you say it is, then don't promise it to be "informal."

Thanks for the guidelines here. I think it is very beneficial for every HR personnel to be familiar with this process.