Coffee grounds are considered a good source of nitrogen and are often suggested as an addition to compost. It is considered as a "green" material and should be layered with "brown" materials. See https://www.bookstore.ksre.ksu.edu/pubs/MF1053.pdf for more info on "green" and "brown" materials.

 Caffeine should not be a problem for the plants but can be toxic to slugs and snails. See below.  What I don't know is how much caffeine is left in coffee grounds after the coffee has been brewed.  

Caffeine and Slugs, Ward Upham, Hort 2011, No. 50, December 13, 2011    

 

Many gardeners have heard of using beer to lure slugs to their death but now it appears we may have another weapon in our arsenal. Robert Hollingsworth of the USDA Agricultural Research Service (ARS) in Hilo, Hawaii was working with caffeine solutions that had shown promise in repelling frogs. They noticed that the 1- and 2-percent solutions also killed snails and slugs. Further research showed the 2-percent caffeine solution more effective than metaldehyde, a common pesticide used to control slugs and snails. It was also found that caffeine solutions with as little as 0.01 percent caffeine reduced feeding significantly on cabbage leaves dipped in the caffeine solution. Instant coffee is about 0.05 percent caffeine and brewed coffee is normally somewhat stronger. (WU)