Reminds me a lot of why I like the main cast in HXH's Chimera Ant arc so much (spoilers for that arc);
They aren't all model-looking people like the cast of characters in a Bleach arc for example
Roshi did some damage to some of Frieza's men in Return of F, I believe. So they may have given Roshi an unexplained increase in power and with his original role as being a very strong and experienced human fighter, I suppose it's fair
And yet they invited Roshi.