Apparently, the group was active 1965-70. The All-Music Guide has this biography:

Although they are most remembered for including Don Williams in their lineup prior to his ascension to solo country stardom, the Pozo-Seco Singers were much more oriented toward pop/folk than country music. The Texas threesome formed in Corpus Christi, Texas, where Williams and Lofton Kline had a duo called the Strangers Two; with the addition of Susan Taylor, they became the Pozo-Seco Singers. Playing cleanly executed, coffeehouse-style folk, the trio could sound like a far more commercial, pop-conscious version of Ian & Sylvia with their conscientious blend of male and female lead vocals and harmonies. Not as hip as Ian & Sylvia or Peter, Paul & Mary, and not as blatantly commercial as, say, the Seekers, the group had a few small pop hits in the mid-'60s, "I Can Make It with You" and "Look What You've Done" both entering the Top 40. After a few albums for Columbia in the last half of the 1960s, the act broke up, leaving Williams to pursue a solo career in the country field.

I didn't realize the group had only three members. I pictured them as being the size of the New Christy Minstrels. Their four albums were:

• Time (1966)
• I Can Make It With You (1967)
• Shades of Time (1968)
• Spend Some time With Me (1970)

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Up above, Jed provided a link to Taylor Pie, formerly Susan Taylor, formerly of the Pozo Seco Singers. There are some nice sound clips at that site. Her voice sounds as terrific as it did in 1966.