Group is the word companies use when several entities sit under one roof: a holding, its subsidiaries and the shared services between them. With Bitcoin in front, it describes a business whose activities all revolve around one asset. The suffix 21 refers to the limit of 21 million coins and gives the name a recognisable signature for people inside the Bitcoin economy. The .com ending matches an international corporate presence.
bitcoingroup21.com is held outside the themed sets and suits corporate use in particular. Likely owners include a holding company with mining, custody and education subsidiaries, a family office investing in Bitcoin businesses, a venture builder that launches several Bitcoin start-ups, or a consortium of independent firms presenting themselves to investors and partners as one group.