Groups can be defined in /etc/subversion/svn_access_control file.
Create/edit the file using following command
nano /etc/subversion/svn_access_control
Use syntax specified as below to define groups and assign members.
[groups]
groupname = <list of users, comma separated>
e.g.
[groups]
myproject-dev = john, peter
myproject-support = maria, cristine
Above example will create two groups named myproject-dev and myproject-support. It will add users john and peter to group myproject-dev and users maria and cristine to group myproject-support.
Groups can then be used to manage repository access