You shouldn'd use the guest group as that might have some unforeseen issues:
Guest in both contextes (MyBB and TS3) means unknown/unregistered users, so everyone not in any other group will be a member of that group.
While that is no big deal without syncing this will become an issue once you sync MyBB and TS3. As MyBB controls every member of a synced group it will automatically remove any unknown member from a group. As the TS3 Guest group is a dynamic group unassigned users will automatically added to that group. But once MyBBs sync task hits it will remove those users from that group, leaving them without any permissions at your TS3 server. So to make it short: dont use the guest groups. I might add a check for that in an upcoming version of the plugin.
You shouldn'd use the guest group as that might have some unforeseen issues: