I saw a post asking this same basic question. My situation is this. I have a body that I brought into blender as a collida, three pieces being head, upper body and lower body. The previous answers I see for something like this say to use ctrl J to join. I even see a button to do this. But when I do it the body parts compress horizontally and become even further apart. Also no one mentions what mode to be in such as object or edit mode. Any ideas let me know please . thank you.
It sounds like you objects may be scaled or have non applied transformations
Joining with Ctrl + J is done in object mode, selecting all objects you want to join and selecting the "receiving object" last.
Try applying all transforms to the objects before joining with Ctrl + A > Apply Rotation, Translation and Scale.
After that also try converting all objects to mesh with Alt + C > Mesh to apply all modifiers and deformations they may have. Only after that can you safely try to join.
Otherwise post your blendfile so we can look at what might be wrong.