*Help* adding user to local admin

Hi there, i have rolled out a server to a friends business and i would prefer not to give the users the local admin password but instead just make them all admins on the machine(this is needed to install apps)

All the users are logging in by machines joined to OD so using the /LDAPv3 node

i have tried the following
sudo dseditgroup -o edit -n /LDAPv3/192.168.1.100 -a Workgroup -t group admin

but getting the message
"Need to specify username and password"

i have tried the following with the local admin username and alsu the users username
sudo dseditgroup -u username -P password -o edit -n /LDAPv3/192.168.1.100 -a Workgroup -t group admin

but getting the message
"Could not add member to group"

Essentially i am trying to add the group "Workgroup" from OD which has all the users in it in to the local admin group on the machine

any ideas??

I have also tried
dseditgroup -o edit -a workgroup -t group admin
dseditgroup -o edit -a workgroup -n /Local/Default -t group admin

however when i check the group membership it is not there
dseditgroup -o read -n /Local/Default admin
dsAttrTypeStandard:GroupMembership -
root
muelleradmin

Thanks in advance