I wish I could help, but I'm not really sure what you are asking.

Is there a specific piece of music that features this that you are trying to play? Or are you just asking in general?

thanks joel--no particular piece --It is something I will run across on the written page--

supposedly -- the tempo climbs at a--steady smooth ratio --

-something a semi player could fake a bit--

The mystery is  H ow can 2 or 3 hit the same next in line tempo --at the same time--

and then it's counterpart--retardation may take it the other direction--

It is done in classical and sound tracks somehow--There has to be a method 
It might not be one --that the guitar style  guys adopted

Perhaps practice?

Banjo music is usually pretty vague about tempo. Changes are indicated with “quicker” or “a little faster” or “rit…” and similar.

Orchestras have conductors that control tempo.

Groups usually have a leader.

We don’t have people playing “backup” or “rhythm”. Instead they play accompaniment.

I think language matters and accompaniment is cued into accompanying the solo.

I can think of one 2/4 tune that I play that includes the instruction to slow the tempo gradually (rallentando). That tune tempo is something like ‘animated’ or ‘lively’, so fairly quick. The rallentando in that tune is only two bars duration.

It’s really just feel. The 'slowed' bpm isn't indicated. We are free to decide how slow sounds appropriate. In two bars there isn’t much scope how gradually we reduce the tempo, because we only have four whole beats in which to complete that tempo change.

Are you asking how 2 or 3 players would keep in time during that tempo shift? I'll second Joel and say that’s simply down to practice/rehearsal.

The ‘a tempo’ return to original tempo is usually precisely indicated in notation. We reach that beat in unison at the tempo we have slowed to. Unless there is additional instruction to increase tempo gradually 'a tempo', the re-set is immediate. From there on we’re back on track. Does this help?

thanks men you pretty well answered it--It would likely be one man setting the pace and the others rehearsing to get in sync asap--
I think I will start with trying to hit 3 different speeds back and forth-- I have digital keyboards where I can program an accelerating practice beat -to work with --It just bothers me to see this used and not tried a hand at it