I can see that i belongs to client group, but when i check /etc/group i don't see the group name there.

Distribution:
SunOS solaris10_t 5.10 Generic_142900-02 sun4v sparc SUNW,SPARC-Enterprise-T5220 Solaris

solaris10_t: ~$ groups
client
solaris10_t: ~$ cat /etc/group |grep -i client
solaris10_t: ~$

what will say ypcat group | grep -i client ?
what is the content of /etc/nsswitch.conf ?

solaris10_t: ~$ypcat group | grep -i client
can't communicate with ypbind


cat /etc/nsswitch.conf
#
# /etc/nsswitch.dns:
#
# An example file that could be copied over to /etc/nsswitch.conf; it uses
# DNS for hosts lookups, otherwise it does not use any other naming service.
#
# "hosts:" and "services:" in this file are used only if the
# /etc/netconfig file has a "-" for nametoaddr_libs of "inet" transports.

# DNS service expects that an instance of svc:/network/dns/client be
# enabled and online.

passwd: files ldap
group: files ldap

# You must also set up the /etc/resolv.conf file for DNS name
# server lookup. See resolv.conf(4).
hosts: files dns

# Note that IPv4 addresses are searched for in all of the ipnodes databases
# before searching the hosts databases.
ipnodes: files dns

networks: files ldap
protocols: files ldap
rpc: files ldap
ethers: files ldap
netmasks: files ldap
bootparams: files ldap
publickey: files ldap
# At present there isn't a 'files' backend for netgroup; the system will
# figure it out pretty quickly, and won't use netgroups at all.
netgroup: ldap
automount: ldap
aliases: files ldap
services: files ldap
printers: user files

auth_attr: files
prof_attr: files
project: files

tnrhtp: files
tnrhdb: files
sudoers: files

in the file nsswitch.conf you can see groups are taken either from file (that is /etc/group) or ldap. in your case the group client is not local, so does not exist in the file, but on the ldap server. (this is valid for users also, there are local users configured in /etc/passwd and global user accounts configured in ldap).

Thanks. I got it.