typically I'd probably say "hi, want to ride together a bit?" (Translation, I'll draft behind you and then I'll pull for awhile.) Then I'd be sure to take a pull in a mile or so, or less, depending on the terrain, and how things were going. If I got no response or a huff or a no thanks, I wouldn't take offense, most likely: Maybe he had a specific training goal or something in mind. Or maybe he is just careful to ride in a line with riders he know can hold their line.

Anyway, riding in a pace line is so much fun and can really take a load off, especially if you've got 4 or more riders. For me, 6-8 is ideal; that way, legs stay fresh! Of course, you want to be with folks who ride in your speed range, or just above it. Have fun!