Like Sal says, we need to find real examples of group marriage to be sure. But I suppose even without such evidence, we can speculate.
I read some meta-study, similar to the one mentioned in this thread, which compared marriage forms in lots of cultures - percent polygyny, polyandry, monogamy, and group marriages. In their definition, a group marriage was distinct from polygamy. The way I understand the difference (or maybe just the way I personally define it) is that in polygamy one person is married to several other persons, whereas in a group marriage more than two persons are all married to each other.
By that definition, when Debra also joins the group, if she is considered married to only Alec and Carl (and Carl is only married to Brenda and Debra) then it would not be a proper group marriage, but instead a case of both polygyny and polyandry. But that distinction might not be necessary to make; a more pragmatic definition is that any culture which allows both multiple men and multiple women in one relationship can be said to allow group marriage.
The study found that the number of cultures which allowed group marriages was zero. But that is of course only traditional cultures, and does not imply that such relationships don't exist.
If we look at group marriages in the sense of all-married-to-each-other, then they can only be accepted in a culture that allows homosexual relationships. As far as I've understood from other studies like this, many cultures do, but most (all?) of them a) don't consider them marriages, and b) don't have equal homosexual relationships; the partners are distinguished either by age, by power, or by some sort of gender roles. I have heard of some cultures where men can not only have several wives but also several male partners, but they take on a distinctly different role and are therefore not really "married" to the women.
So the only culture (I know of) where proper group marriages could potentially be acceptable would be our very own. And it does exist, even though it is not exactly completely accepted in the mainstream culture.
On a side note, I've been told that there was a suggestion a few years ago in the Swedish parliament that marriages of up to 13 people should be allowed. If I'm not mistaken, it would have made us the first nation to allow two men and two women to marry. Sadly it failed, and our new marriage legislation is only the watered-down version: Gender-neutral, but nothing else is new.
It was also suggested that marriage in the legal sense should be separated from the religious sense. Quite rightly, I think, because other groups of cohabitants (siblings, etc.) should really have the same legal rights as people in a sexual relationship. The smaller churches favoured that idea (because it would allow them to say no to gay couples) but the former state church opposed it, so that failed too.
Thus, the only group marriages I know anything about, and can try to answer questions about, are the legally unrecognised ones that exist in the sexual liberation subculture in our own culture.
1. Most such groups would be rather politically progressive, and would consider the right to sex as a reactionary idea. (That is, unless the relation also happens to have a BDSM component; such relationships are relatively common in the BDSM community.)
As for denying the right for a spouse to have sex outside the group (known as "polyfidelity"), it certainly happens, but I think a person who is okay with group marriage is also more likely than others to accept promiscuity. In other words; no, it is acceptable, but not more acceptable than in monogamous relationships.
(Concerning the observation that those rules no longer hold for mainstream marriages: I think that is definitely just a sign of the legal status of marriage losing power. It's not that we consider violating those rules acceptable, it's more that we don't consider it a matter for the court.)
2. The most common kind of group marriage is that of only three people. In that case, the two of the same sex sometimes consider themselves to be married to each other, and sometimes not. It is probably more common if they are two women, since bisexuality is generally considered to be more common in women. For larger groups (and same-sex groups), I would say that the common view is that they are all married to each other.
But there is another interesting thing to note. In traditional polygamous cultures such as the islamic, it is clearly stated that a man should love his wives equally. But in modern poly/group relationships, a common view is that there is one primary relationship, and then you add on a secondary. If the relationship should not work out, the secondary partner will have to go. But in most cases the secondary partner would probably not be considered married.
3. I've never actually heard of that happening, not to the extent that the people involved have called themselves married. But it is not too unusual for two couples to form some sort of sexual relation to each other. I would say the likely possibilities are
a) the couples "marry" each other, as corporate entities
b) the individuals all marry each other, dissolving the original couples; just like any other four-people group marriage, the same-sex persons may or may not consider themselves married
c) the two members of one couple marry one person in the other couple, making it more or less like a strict polygamous relationship
4. That would be 105 marriages in just four people... but maybe they like wedding parties.
4a. In a group which has agreed to polyfidelity, the norm would be that you have to have the permission of all the other group members. In an MMFF relationship with people who are predominantly heterosexual, it might be that Brenda doesn't care as much about what Debra does, but it would probably not be okay to go against her veto.
But other possibilities exist; if we have a couple+couple type relationship, you might only have to ask your primary partner. In a one-to-many relationship, you might only have to ask the one.
4b. It seems extremely unlikely that any egalitarian group would force any of its members to have sex.
4d. While such democratic structures of group marriages sounds like an interesting idea, I have not heard of anything like that in real life.