@Turquoiseraindrops
As posters before me have said, your friends would need to be in a committed relationship, in the eyes of the Home Office. To them, that means marriage. I totally understand how hard that can be when they have not been in each other's physical presence for more than visitor's visas time allowed to make that kind of commitment but that's what those "family visas" entail. They would need to apply for a fiancé/ fiancée visa.
I know this because I was on a fiancée visa to the UK back in 1995. Back then, you had to demonstrate a proven long-distance-relationship over three years prior to application. If I add all the days we spent together physically within those three years, they amounted to 45 days. The only way we could be together permanently was to take the plunge and get a fiancée visa. We married 10 weeks after I arrived.
You don't have to wait three years now, you only have to have met once, but it has a long list of requirements. Your friends will need to go on gov.uk and find the visa they need. They will have to prove their relationship through two photographs (only two) of them together, they will need to show airplane tickets of visits to see each other or itineraries and communications between them like screenshots of texts to each other and emails over the entire duration of their relationship, no matter the length.
If one of them is already in the UK, it's a bit easier. They will have to provide proof of accommodation (land registry or rental agreement) to make sure that the residence wouldn't be crowded. As for employment, the UK resident would need to prove that they have employment prior of six months of the application and that the employment pays £18,600 p.a. Those are the big requirements with little steps in between. The visa would need to be done whilst the applicant is still in the US! This is important.
I found that there are two big sticking point for that specific visa is that the applicant would have six months to marry. It's was a bit tricky before coronavirus but now it's a bit more complicated. They'll have to check out the details from their local registry office or place of worship or wherever but they need to get the ball rolling because there is pressure with the time. The bigger one is that on a fiancé/ fiancée visa, the applicant cannot work or volunteer in any capacity until they are married AND they cannot access the NHS until they marry and change their visa from Fiancé to Spouse. The applicant really needs to have their own insurance for the duration of the visa. Remember, they pay for a fiancé visa that's valid for six months and then right after, they pay again for a spousal visa that's good for 30 months. That's pricey.
Personally, if the couple is dead serious about each other, I would recommend that when it's possible, the UK resident goes to the US and they marry. As soon as they get their marriage certificate, they apply for a spousal visa. That way, they only pay for one 30 month visa (the process for "Indefinite Leave to Remain" is 5 years), when they're approved, they live together and don't have to worry about things for another 30 months and they can work, as soon as they get their NI number and they have the ability to access the NHS if they need to.
Moving to the UK is not for the faint-hearted or those who are cash strapped. It's a hell of an investment but hopefully, it's worth it. They could also apply to marry and move to the US, it's faster and cheaper.