My adult boys have been going out since (racks brain) about a month or so ago. Make sure the grass is DRY. If you can have your run in sunlight at the moment to make sure it's completely dried the grass. Make sure there's no dog/cat/fox pooh. Buttercups , bulbs, dock leaves are poisonous. My boys avoid them but a piglet might not so don't risk it, dig it up. Put a couple of boxes in (with two cut out doors)and some cardboard inside to sit on. Check for staples and sellotape though. You can put a cover over part as a windbreak, they don't like draughts.
Start them slowly. 20-30 minutes is ample at first. Built up slowly. Baby guineas especially can get bloat . My boys have an hour in the evening if it's warm enough or 2-3 hours at the weekend but I'm not putting them out till afternoon when I know it's dry.
Don't put them out every day at first though, rich spring grass can give them loose pooh.
You could start by cutting some grass to begin with, let them sample it. (I give mine a plate of grass if they don't go out . But if they have grass time I give them a small supper with unlimited hay and some pellets)..