Erm i think the command you might want to be using is /pex user {name} group set {group_name} That should fix the problem
Yeah like Kayota said you need to have default: false on all the other ones so pex will work.
Did you use essentials.warp and essentials.warps.<warpname> together... so like groups: default: default: true options: rank: '1000'...
Separate names with a comma.