Advice needed on integrating a mare into a group of geldings!

If anyone out there has had experience introducing a mare successfully into a group of geldings, I would very much appreciate knowing how you did it.

I have a friend's mare coming to retire with me at the end of the month, and am anxious to ensure it goes smoothly without loss of limb or life. The three geldings run as a mini-herd because I've set up the barn so all four doors open up into a blue-stone area and from there to the fields. The stalls double up as their run-in sheds, and the three fields interlink so they can come in any time when they want.

This system has been working brilliantly, and I need the new mare to fit into it for her stay to be possible.

Obviously I have to work up to her being allowed in with all three boys. But I'd be interested to know if anyone else has done this before and if so, how.

Thanks to anyone who answers this post!