I mean, for a time due to special circumstances out of his control, Elio was a Team Skull Admin that was inducted on Lusamine's orders because she wanted to keep Elio properly bound for a certain duration while he was doing missions on her behalf - some of which required him to work with Guzma. An additional effect is that Guzma would grow stronger in his right when hanging around Elio, turning the former into one of Lusamine's most capable pawns in the long term.

So, in a way, Elio has a simultaneous membership with both the Aether Foundation (unofficially) and Team Skull. Only a select few higher-ups in the Aether Foundation knows about Elio's circumstances and his double-membership - with one or two of them being directly involved in some way.