Quick Answer:   It sounds crazy to say that grass can be dangerous for your horse but it can. New green shoots are full of sugar and sugar in excess is heading for laminitis.

All winter you have been dealing with a lack of grass, probably a lot of brown dry stuff since the summer has been so dry.  We have been feeding a whole lot of hay and that has been and still is, very expensive. So it can be tempting to throw your horse out on to the green to save you some of the costs, but beware, vets cost a lot too.

Faced with a hungry horse and a newly green sward, keep it all gradual as best you can. There are lots of options:  Restricted grazing hours.  Strip grazing.  A track system with no grass. A grazing muzzle.

Given that horses need friends as well as food, any of those options allow them to stay with friends but if the only option is the full field with others who don't need restrictions, then a grazing muzzle is the only solution.

How do you know if your horse needs restricting and they don't?

Previous laminitis episodes - a big pointer to restriction.
Being overweight - my Skye was overweight but I didn't see it, be aware.
They live on "fresh air"  and their friends don't.

So the upshot is to notice, stay careful and restrict where needed.