If the boat your coming alongside is very large, which I'd assume it is, get a good length stern rope onto her and keep some power on. This will slow down the movement, and I'd assume that the "mother ship" is fitted with DP so she wont move off station whilst you do this.
I've been out on some crew transfers to warships carrying out sea trials and it is much easier going alongside when both boats are moving, however when they are at anchor we put up 2 stern lines, in case one breaks mid transfer, and keep the outside engine ahead, this slows down the pitching and we have carried out transfers in a F8, the boys like to come ashore
The boat does tend to swing at her mooring ever so slightly, but by the time it becomes a prob we have left.
Andy