See also the official documentation:
Help Center > Group Owners Manual > Inviting people to join a group > Inviting more than 20 people at a time.

Why Can't I Invite More than 20 People to My Group?

Well, you actually can....but if you Invite (or Direct Add) more than 20 people to your group in a 24-hour period, you will likely receive this message:

You have reached the limit for number of invites that can be sent out without approval. Once your new invites have been approved, they will be sent out. You will receive a notification when this happens.

See https://beta.groups.io/g/main/message/7589 for an explanation of why this delay/review exists. Yahoo groups had a similar requirement, designed to thwart those who would set up a group and use it for spamming purposes. This "delay for review" applies no matter which plan you are on -- free, premium or enterprise.

To be totally clear, this is not a "limit"...you can send as many invitations as you want. However, there will be a waiting period while groups.io management reviews the list of invitees to ensure you are not a spammer. The remaining invitations usually go out within a few hours. If you wish, you can go to Admin>Invite and click on the "Sent Invitations" button at the top to check status (or wait for the promised email notification).

Generally, it's highly recommended that you send all your invitations at once. You can do this by uploading a plain text file (not CSV, not Excel), containing all of the addresses, one address per line (and nothing else). Or simply cut-and-paste all of the email addresses into the Invitations box.

There is no advantage to sending invitations in 20-person batches. Doing so will only incur additional delays.

Last updated:  19OCT2019