Ralph,
Getting visa at any US Embassy around the World is an ordeal for us,
poor citizens of "failed states".
I personally was never denied a US visa, but my trips have been work or
business related. However, I saw many times, and heard from numerous
people including close friend about "unreasonable" denials of visa
that they experienced. I saw Belorussian mothers in the embassy in
Minsk not getting visas to visit their children living in the States.
There is no rule of thumb for getting it. Basically, the people you
invited have to prove to the Consul that they are not intended to stay
in the US longer than is permitted by the visa they are applying for.
They have to provide documents confirming that they have "strong ties
to the home country". Things that count are: property in Belarus,
saving accounts, relatives living in Belarus, especially children.
Also, they have to show documentary evidence of how big friends they
are with you - the Inviter.
The assumption is that we, poor devils, always lie, and our goal is to
sneak in the "better World" by any means. Therefore the evidents should
be substantial. Recent photographs picturing them and you, personal
letters (I'm not kidding), copy of your bank statement, copies of their
bank statements, employment letters. If one of your friends is a female
her chances are very low. If she is single - forget it, she'll never
get a visa. (Those slavic wenches -always looking for rich husands).
The only chance that can potentially buy her a visa is if she has a
little child, or better many children, and they all are going to stay
home while she is looking at the marvels of the free world. If some of
the invitees happen to be unemployed or can not show convincing proof
of employment, their visas will be denied too.
It's not a walk in the park - getting an American visa, especially now.
For me personally this process has always been invariably discouraging
and even humiliating. I've never been comfortable with the idea that I
have to start the conversation with a consul by having my fingerprints
scanned.
As for the "letters of support written by Senators" - sounds to me like
something that the Consul should appreciate. Not sure if it's a
bipartisan though. Maybe wrong Senators?
So, the bottom line is, yes getting visa is a difficult task, and
chances are rather low. Good luck to you and your friends.