There are a number of 'groups' implementations in the fediverse and they all work slightly different from each other. There are important reasons for this having to do with group permissions and privacy - and the differences are unlikely to be resolved any time soon. However, If my recent pull request to Friendica is merged, you will soon be able to select a group from Zap's drop down "permissions" selector and post to any of them without knowing the implementation specifics. This will work with Zap/Redmatrix/Mistpark/Osada groups that require posting to groups via DM, Friendica forums and gup.pe groups that require a mention (Friendica can use a 'bang' mention as well as an 'at' mention), and Hubzilla forums that typically require a bang mention. On the short term it will require either the next releases or development branches of Friendica and Hubzilla to work seamlessly with groups/forums in those projects.