The "Invite" button doesn't appear on an operation? For a group, only one condition matters: the operation must have been created by your group.
Condition for inviting: the operation belongs to your group
Only the entity that created the operation (its owner) can invite pharmacies.
An operation created by a laboratory can't be invited to from your group account: the laboratory does the inviting.
For a group account, no permission is needed to invite: the "Invite to operations" permission only exists for laboratory accounts, so there is nothing to request from your group's administrator.
Inviting pharmacies once the condition is met
Click "Operations" in the left-hand menu, then click the operation's row: the operation panel opens.
At the top right of the panel, click "Invite".
Choose the "Pharmacies" or "Mailing Lists" tab, tick your selection, then click "Save".
Result: a message says the invitation request is being processed; the invited pharmacies then see the operation in their operations table.
Another way: right after you create an operation from a page of the new interface, the summary of the created operation offers "Invite to the operation".
Why this path: the operation panel opens for any existing operation; the summary only appears once, right after creation.
If it doesn't work
The operation is indeed your group's, but the "Invite" button is still missing → the operation may have been saved under another entity's name; contact support via the Opeaz bubble, giving the name of the operation.
Still stuck → contact support via the Opeaz bubble, stating the name of the operation and the pharmacy's CIP.