Architecture
Guest accounts: the population nobody removes
External collaborators accumulate, projects end, and nobody deprovisions. What a guest can actually see, and the review that fixes it.
Microsoft
Guests are created by everyone and removed by nobody
Adding a guest is a self-service action available to ordinary users in most tenants. Removing one is nobody's job. The result is entirely predictable: a population of external identities that grows monotonically and is never reviewed.
In tenants a few years old this is routinely hundreds of accounts, a large share belonging to people who left the partner organisation long ago.
What a stale guest can still reach
Whatever they were given, which is usually more than anyone remembers. Teams they were added to, the files in those teams, and any SharePoint site shared with the group. Guests also frequently sit inside groups used by Conditional Access exclusions, which is how a dormant external identity ends up outside your access policy.
The lifecycle that actually works
Set an expiry at creation, not a review later
Access packages in entitlement management can grant guest access with an end date. Time-bound by default is the only version of this that survives.
Run access reviews on the guest population quarterly
Entra ID access reviews can ask the sponsoring internal user whether the guest is still needed, and remove them automatically if there is no answer.
Block guests from being added to sensitive groups
Particularly Conditional Access exclusion groups and anything granting administrative scope.
Remove rather than disable
A disabled guest still appears in group memberships and sharing dialogs, and creates the impression access exists. Remove them.
The verdict
Time-bound guest access with quarterly reviews
It makes deprovisioning the default rather than a task somebody has to remember, which is the only reason this problem exists.
Do not solve it by blocking guest access. Collaboration moves to personal email and consumer file sharing, which is strictly worse than a governed guest.
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