What's happening
If an event, or a ticket type, is restricted to one or more Groups, only people who are logged in and whose contact profile is in one of those groups can see it. Anyone else, including a member who hasn't created a TidyHQ account yet, will see a 404 (page not found) rather than a login prompt.
If a member can't see the event
Are they logged in? Ask them to log in or create an account first. They must use the same email address, first name and last name that are on their contact profile in your organisation (an exact match), so TidyHQ can match them to it.
Are they in the group? Check their contact profile.
Do they have duplicate profiles? Access is checked against the contact profile they're logged in as. If they have two profiles and only one is in the group, either add the correct profile to the group or merge the duplicates (Contacts > Possible Duplicates).
Finding who still needs an account
Add the TidyHQ Account Email column to your Contacts table. Contacts that have an email address but no account email haven't signed up yet. A Smart Group of the event's group members with this column empty gives you a list to email with sign-up instructions. Put those instructions in the Smart Group's description so other admins can follow them.
Members with no email address
They can't create an account to log in themselves. Add them manually with Add Attendees. Note a ticket can't be delivered without an email address.
Alternatives
Sharing a ticket passcode with the whole group is possible, but anyone who has the passcode can use it, so it undermines the group restriction.