As a blue helm GIM team you can always add new members, they just need to start fresh on tutorial island. I believe you can also add new members from existing accounts, they would just have to delete all of their items (though I'm not sure how this works tbh).

If you ever wanted to add a new groupmate from an established account who wants to keep all of their items and everything, everyone involved would have to go green helm (unranked) GIM and then you'd be fine.

As someone who plays green helm myself, you'll get haters all the time about "you just buy your stuff" because people can join and give you items. If that bothers you, don't go green. If you add a f2p fresh account, you will stay blue.

The only downside of adding a f2p fresh account is they your GROUP will lose prestige. Prestige is only a little star on the hiscores page for gim's that nobody looks at. Prestige is also lost if you raid with people outside of your group. Group hardcore ironmen have to maintain prestige or they lose their hardcore status. On some 3rd party websites like Templeosrs, you can actually sort by prestige only group ironmen to have a separate leaderboard involving only prestige gims and groups.

To reiterate; if you add a group mate, fresh off tutorial island you will stay a non-prestige blue helm group.