Need to add traction to existing polished concrete surface
You can make the surface a bit rougher by using an acid etch. Muriatic acid &amp; water is commonly used to clean concrete prior to sealing or painting. A strong solution allowed to sit will actually etch the slippery surface away. It's much easier than grinding. 

Once the surface etch is deep enough rinse with fresh water if the first solution did not etch enough add a higher percentage of muriatic acid. I would start with 4 parts water to 1 part acid. Remember to always add acid to water.

Most big box stores sell muriatic acid for this use and pool supply stores have it for balancing the pH in pools and spas so it is easy to find and not very expensive at ~$10 per gallon.

Added per comment: we always add acid to water to prevent an exothermic reaction. If water is added in some cases it causes the water to rapidly heat and can boil and splatter the acid from the reaction. So to be safe remember AAA always add acid