DMR, as I've come to view it, is largely restricted in function by the subscriber radio. The repeaters are pretty dumb and will pass anything in most cases. It's up to the radios meant to use it to have the functions set up precisely to make the system work properly and limit access to the most important settings: frequency, color code, time slot, talk group. FPP makes some of that accessible, but that isn't an option in use in most radios either.
If you want a DMR radio that is able to access groups manually, a radio with OpenGD77 or an MD380 with MD380 tools - both tailored for ham use, is probably the best route.
-Josh