As well as opening a shift to everyone, you can send it to particular workers.
Open the shift, or go to Staff.
Choose Invite.
Pick specific workers, or invite all your favourites at once.
Send — they're notified and can accept.
Favourites get first refusal. Workers you mark as favourites are prioritised for invites and for auto-fill, so your most reliable people hear about your shifts first — more familiar faces on your floor, and higher fill rates.
To favourite a worker, open Staff or their profile and mark them as a favourite. If you'd rather work with a fixed group of workers only, see the article on working with exclusive workers.