What makes a field not groupable

I'm not 100% sure what makes it "groupable". The Considerations When Using GROUP BY does mention it:

Some object fields have a field type that does not support grouping. You can't include fields with these field types in a GROUP BY clause. The Field object associated with DescribeSObjectResult has a groupable field that defines whether you can include the field in a GROUP BY clause.

Although not exactly elegant, you could try to copy the not groupable field to a groupable field via a workflow rule and then group by that one instead.

It appears that you can not group by custom number fields and not the ability to group by formulas is a request on the ideaexchange so you will need to create a workflow rule that copies the number to a text field.

Note that most standard number fields cannot be grouped by as well but a few like account.numberofemployees can be.