you definitely can use stems in a tincture. You probably won't have to mess with a coffee grinder, although it really won't hurt your chances. Essentially you are going for the trichomes off the stems anyway, which will be fully exposed regardless of whether or not you ground it up, and you may lose a lot of the trichomes in the grinding process.
Make sure you decarb; this is the most important part of the whole process. Let it sit in an oven at 325 for 5 minutes or so. If you aren't cooking it in alcohol, let it sit in alcohol (again, everclear is supreme for this) for at least 2 weeks.
Eric the ruler's videos are great, and they are also absolutely hilarious. Recommended for 2pac fans.