I am sharing my Google Sheet as "anyone with the link can view." The spreadsheet contains grouped rows. It appears that viewers cannot click the plus sign icons to show rows that were hidden by grouping them. How can the viewers expand the grouped rows?
Users who have view-only access to a spreadsheet cannot modify it, and expanding rows counts as a modification, so it simply does not work.
One workaround is to share the spreadsheet as "anyone with link can edit", and protect individual sheets or ranges within the spreadsheet as required. If you are protecting full sheets, you will have to use the Show warning only option because otherwise the sheet will be completely view-only and row grouping cannot be used.
Another alternative is to use filter views instead of row grouping to hide and show rows. This requires that some column in the dataset contains tags that let a filter view decide what to show and what not. See the Filter views example spreadsheet for an illustration.