Yeah, there are a lot of websites you can search for ideas. Some other slogans I've heard are:
No matter the letter, Greeks do it better
Letters for a Lifetime
Letters today, leaders tomorrow
Individually unique, together complete
Greeks may stand out, but we never stand alone
Letters aren't a label - they're a lifestyle
(Of course, I took a lot of these from Recruitment shirts, so they might not all work.)
You could put a slogan on the front then put everyone's names on the back. There are so many possible ways to do that.
__________________
On the heart of each sister
lies one 0-----,, that binds us
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