Fully aware this is an old question, but I wanted to point other people with similar issues to some open sourced API documentation run by the community (I’m one of the contributors, full disclosure). It has nearly full coverage of the GroupMe API.
The relevant docs regarding GroupMe Topics are here, but I’d encourage anyone who’s interested to take a look at anything and everything else, because it’s a lot more complete than the official docs at dev.groupme.com.