Is it OK in hobby activities to send email with the names of all the recipients displayed in the recipient field?

As a rule, it is not. The names of recipients must be written in the blind carbon copy (BCC) field so that other members cannot see them, unless there is a special reason for showing the contact details. This follows the principle of data minimisation. You should keep in mind that some email recipients could have a justified reason for keeping their email addresses out of public knowledge. The recipients of the message may not know about such reasons, nor do they need to. For example, if the relations between ex-partners are strained, one of them may need to keep their email address secret from the other. As another example, someone in a significant public position could have their private email flooded with messages if it became generally available. Lastly, we should also keep in mind that everyone is entitled to privacy and should have a say in who gets their contact details.