Shared Channels in Microsoft Teams are much more than external collaboration

PeterSE2023 If you have the Azure AD P2 Entitlement Management feature you can sort of do this, it's the tool that creates an approval and reattestation cycle for membership for a group.

1. Using Azure AD b2b Collaboration sharing if you apply EM to the group your defined approver will need to approve each new external guest

2. You could add a group to limit the scope for the B2B Direct tenant access policy, and I assume then use EM to manage the group, would be a bit awkward as your staff would need to apply to be a member before being invited. Not quite what you were asking for.

I would suggest having a Form / Helpdesk process for staff to request creating the Cross Tenant Access Policy with another organisation, and also assign sensitivity labels to Teams that limit whether they can have external members.