Entities can only be connected to one Group at a time.
If you no longer want an Entity to form part of your group, you can choose to simply Archive it. See this helpful article for more details on how to do so.
However if you need to move one Entity in your Group to another Group, then you will need to contact us at support@getmayday.com.
Note: If you are creating a new Group and see an Entity in the list greyed out and unavailable for selection, this means it is already connected to another Mayday Group. If this is an error, it might be worth reaching out to your team to understand who has added the Entity to their Group, but if you are still unsure then please reach out to Mayday support.