Is it safe to add yourself to group 82 (_clamav)
For your purposes, it's best not to add yourself to an existing group.

To discover what groups are on your computer you will find most listed in /etc/group - less /etc/group will give it to you. You can create a new group by adding it to this list. man group will explain the format of this file. I usually create a group with an id from 600 to 699. Apple seems to use ids above 700 for special groups not listed in /etc/group.

To discover what groups you are already a member of you can use the groups command. To list the ids of the groups you are a member of you can use the command id -G.