Would you get involved

No.
Have the two already bought it? I presume not?

Apologies for not making it clear - yes they have bought it

Not flipping likely!

Has it been made clear what they hope to achieve?

Is there any chance they will go on to sell it to a developer if people don't get involved so they can recoup their costs. It sounds very likely in my opinion or why would they bother?

I would expect there to be some kind of project under way and any involvement would depend on what the plan is for the land and whether it is for a community project. There probably is some kind of plan but it would seem their publicity is sadly lacking.
Samie your friends should find out what it is all about as that does not commit them to anything.
I am a bit bemused that your friends are "almost frightened to go out of the house for fear of being approached by these people" ! What on earth are they are afraid of? Once they know what it is all about then a simple yes or no should suffice I would have thought. After that it would either be the end of the matter or possibly the beginning of a new interest and getting to know different people. It sounds more like talking about 'fear' to be interesting and nothing of any moment so I wouldn't worry until you know there is something to worry about.

PS. If you are worried for your elderly friends then perhaps you should do the legwork and find out what it is all about Samie

Sounds like the people who bought the land now find they can't afford it. They should have found out more about it before they bought.
Your friends should stay well away. They don't seem to have any obligation.
Some people, the chutzpah!

No way, it's up to the two who bought the land to pay for it's upkeep!
In fact, if your poor friends and their neighbours can't go out without being preyed upon it's the police who should be called upon.

I agree - it sounds like harassment!

Definitely not, they bought it so it's up to them to pay for the upkeep. Nothing to do with anyone else.

Perhaps the two who have bought it want to form a consortium, get planning permission and build on it?
It seems an odd way to raise capital, to go round harassing elderly neighbours.
.

It's a definite no from me too

Thank you everyone for your comments