The K-1 visa is meant to be used by a United States Citizen (USC) to bring his/her fiance(e) into the U.S. conditioned on the fact that they will marry within 90 days of entry.

There are some basic requirements to be eligible to use this visa;
1. The petitioner must be a USC. A lawful permanent resident or green card holder cannot petition for a fiance(e).
2. The parties must have previously met in person within 2 years of the filing of the petition, and intend in good faith to marry within 90 days of entry.
3. The intended marriage that would take place must be legal in the sense that both parties are of the age to marry and if any had been previously married, there has been a valid divorce.
There are also other considerations that are not the focus of this writing such as the fact that the applicant must be generally admissible, that the USC’s criminal history may be an issue, and matters related to repeated filings.
The K-1 visa process is attractive to many USCs because it is thought by some to be a quicker way of bringing in one’s would be spouse into the U.S. as against getting married in the foreign country and going the route of an I-130 filing, and consular processing of an immigrant visa. (These days it is the writer’s view that speed should not be a consideration in whether or not to go this route because of the fact that processing times for an I-130 is comparable to that for an I-129F, plus factoring in the fee for adjustment of status in the U.S.).
The K-1 visa would also be the visa of choice where the parties prefer to have their marriage and wedding done here in the U.S. especially if they have more friends and family here.
Pitfalls of Customary/Tribal Engagement
As stated earlier, for a successful K-1, the petitioner and applicant need to show among other things that they have met in person within 2 years and intend to marry within 90 days of entry.
A customary or tribal engagement may be very fluid in the sense that one may not be certain when the parties cross the line of engagement into a customary marriage. Sometimes no two experts or courts would agree as to exactly when a customary marriage has or has not occurred. Where  for example, bride price is the corner stone of a customary marriage, sometimes some local courts or experts have found that there is a valid customary marriage though the bride price has not been paid. Some experts or local courts are prepared to recognize a valid customary marriage where the bride went home with the groom after the initial formal customary engagement. Therefore an elaborate customary engagement may actually be interpreted or misinterpreted as a valid customary marriage thereby making the parties ineligible for a K-1 visa.
It is my opinion that parties who may face this type of situation should avoid an elaborate customary engagement as a consular officer may find that a marriage has already taken place.
The law only requires that the parties have met physically within 2 years, and that they have a good faith intention to marry within 90 days of entry. Emails for example may be a way to prove an intention to marry without the risk of a consular officer thinking that a marriage has actually occurred. In this age of social media, such an intention can also be shown using such media.
Where the parties decide to have an elaborate customary/tribal engagement notwithstanding, it may be a good idea to be proactive and obtain affidavits from the tribal leader(s) or custodian affirming that what had transpired was an engagement only, and stating the reasons for such conclusion under the particular custom.
In no way am I advocating that anyone should abandon their culture out of fear of what a consular officer may decide. The fact of the matter remains that the lines are sometimes not very clear as to what has happened. Ultimately the parties will decide what and how they wish to proceed.