Lord Krsna tells Arjuna : Those who claim to be My devotees are not My devotees but the devotees of My devotees are the best of My devotees.

Why did Krsna make a statement like that?
It's quite radical isn't it?

What does to mean to be a devotee?
Dedicating yourself to the service of the deity. Bhakti is very personal. Normally a person would want to worship only the Supreme Lord, only His deity, Lord Sri Krsna.

But how can you dedicate yourself fully to a person if you do not know about him fully?
If there is something that you are unaware of about him then your knowledge about him is incomplete. Then you cannot dedicate fully to him.
You can dedicate to him only to the extent you know about him. So, Bhakti is not only personal but social too.

Say if you see a friend then just by seeing him you can know about his name, form Nama, Rupa etc but what about his guna and Lila?

Krsna is said to be completely known by knowing His Nama, Rupa, guna as well as Lila.

Nama and Rupa are easily accessible but what about His guna and Lila? How can we know His guna and Lila?
By seeing His dealings with His devotees ( Lilas) , we can know about His guna, qualities.

That is why devotees should desire to see lord Krsna always in association with His devotees. Only if there are devotees will there be dealings between the Lord and the devotees. That's when we can know about Him more and more.