Inviting a trustee who isn't a leader
Many trustees aren't leaders: a parent who's an accountant, somebody from the community. On the Members page, choose A trustee who isn't a leader when you invite them.
They can then do everything in Trustees and see nothing in the other apps: no camps, no young people's details, no bookings. They can't invite people or manage the group. The Members page shows them as Trustee in every app.
It's a starting point, not a cage. The owner can give a trustee more on the Members page, for example letting the Treasurer see Fundraising, the same as for anybody else.
Anybody who leads in Trustees, including a Trustee member, can invite more trustees from The board, and see and cancel the trustee invites still waiting. They can only invite trustees, never a leader or anybody into the other apps; that still needs a Leader or the owner on the Members page.