Issues with crossplay

Is it another xbox one player of xbox PC player? They may need to update their game or windows/graphics card driver if so.

Could also be a privacy setting issue, have them check to make sure have "can play with people outside of xbox live" set to "allow"

This setting is under your profile privacy setting, xbox one/windows 10 online safety.

It's another Xbox one player. Again me and one other person are on switch and we were able to join in just fine so I don't think it's an issue of allowing crossplay.

Can you tell me your communication and multiplayer privacy settings?

Each Xbox users privacy setting only affect them or if other people can interact with them, so your settings are set correct but that doesn't mean theirs is. That being said it could still be a different problem if they have the right settings.

Maybe have them check to see if their NAT is open, the player that cant join may need to open a port on their router if they have a restricted NAT

Crossplay is down to the individual developer's of the game in question. In the case of Microsoft Studios games, then they mostly (far as I am aware) have it due to how Microsoft is running it's gaming side now ie integration on all platforms. Beyond that, each Studio or publisher makes the decision themselves hence why some games have full crossplay and some have almost none.

Is it another xbox one player of xbox PC player? They may need to update their game or windows/graphics card driver if so.

Could also be a privacy setting issue, have them check to make sure have "can play with people outside of xbox live" set to "allow"

This setting is under your profile privacy setting, xbox one/windows 10 online safety.