See also the official documentation:
Help Center > Group Owners Manual > Locking and unlocking groups and subgroups

Locking a Group

[Note: Only applies to Premium or Enterprise groups.]

It is possible to lock an entire group (or more frequently, a subgroup). Locking a group makes its content read-only. This is analogous to locking a topic -- the messages in the topic are still there, and you can still read the messages and view/download attachments and so on; but you cannot delete/edit/add messages or make other changes. See https://beta.groups.io/g/main/message/16776 for details on when this feature was introduced. 

There's a baby-blue "Lock Group" button at the bottom of the Settings page. Clicking this button and confirming in the resulting dialog box initiates the lock. The read-only/locked status applies to everybody, including Members, Moderators and Owners. Locking is not permanent...any group Owner (or a Moderator with the "Modify Group Settings" permission) can unlock the group/subgroup at any time. Please also note that groups on the Basic (free) plan cannot be locked.

Once the group is locked, you can: 

• Look up already-existing Calendar events
• Review Poll results
• Read everything in the message archive
• Download Files and Photos
• Review member permissions
• Etc.

You cannot: 

• Create new Calendar entries or RSVP
• Create or add your response to a Poll
• Post any new messages
• Upload/modify any Files and Photos
• Change group settings or member permissions
• Etc.

If a Member attempts to post to a locked group via email, the active Member Notice of type "Locked Group" is returned. If no such Member Notice exists, a more generic bounce message is returned:

">>> groupname@groups.io (reading confirmation): 500 This group is locked and does not allow posting."

Note that 500-level bounces usually indicate a permanent condition. If you intend to lock your group for only a short period of time, creating a custom Member Notice is highly recommended.

The original need for the "lock group" function is unclear...no posts exist in beta requesting it. One possible use is to close down a subgroup that is no longer needed while retaining its content for future reference. If you're reading this, perhaps you have other ideas on how this feature might be handy.

Last updated: 28MAR2020