After performing an action to add/remove/copy/export to a group, your subscribers will appear grey for a while.
This means the action is being processed, which usually takes just a minute (or slightly longer, depending on other factors).
To proceed, please wait for a minute or two and refresh your page. Then, it should be resolved.
💡 Tip: you might need to clear your browser cache in case you still see subscribers greyed out after a while.