A dashboard in Atwork is a Charts view on one sheet, so it follows that sheet. Anyone who can open the sheet can open its dashboards and sees the same numbers. There is nothing to share separately: add someone to the space, and the sheet's dashboards come with it, unless the sheet has its own access list.
Open a sheet, click + after the view tabs and choose Charts. Every widget on that dashboard reads the same sheet.
Everyone who can open the sheet, whatever their role on it, which usually means everyone in its space. Someone who isn't in the space doesn't see the sheet or its dashboards in their sidebar.
If a teammate can't find a dashboard, check access in the same order as for a sheet: the workspace, then the space, then the sheet. See Why can't my teammate see a sheet?.
Yes. Widgets count the sheet's records live, for whoever is looking, so a Viewer and an owner see the same totals. The exception is a filter that uses is me, such as Assignee is me, which shows each person their own records.
Space roles are Admin, Editor, Operator, Viewer and Filler. See Roles and permissions.
Admins also see Publish in the view tab's menu. It turns the view into a standalone app with its own link and its own list of people who can open it. Check that menu before you assume a dashboard is only seen inside its space.
A Chart or KPI number tile on a space home shows its numbers only to people who can open the sheet behind it. Everyone else sees the tile's setup steps instead of numbers. If the tile tells someone "The selected sheet no longer exists", the sheet is usually still there: that person just can't open it. See Embed a dashboard on a space home.
Related: Dashboards and charts · Why can't my teammate see a sheet? · Roles and permissions