You might get a few different stances on this.
Personally I think that a married couple can do pretty much what they like to please each other, as long as it brings them together and doesn't harm their relationship with each other or with God.
However, the Roman Catholic stance on this is that sex is primarily for reproductive purposes, hence any sex that does not involve actual intercourse is sinful. Also, Roman Catholics disagree with the use of contraception.
All Christians, however, agree that sex outside marriage is wrong. Sex is one of the things that brings a husband and wife together and helps them through the tough times (and there will be tough times if you're going to stay with someone for the rest of your life).
Because of this, that's definitely a "no" to group sex.
Hope this helps.