Hi Sebastian, the Join command is the regular way to knit together surface edges - if the model is connected in other places so that it's already one piece you can try using the Separate command to break out the surface first before rejoining it.

But surfaces need to have edges within 0.005 units of one another to be joinable.

If they are fairly far apart from that amount then they will probably have to be remodeled. If they are only just barely apart from that you can try scaling the model down by one tenth in size and then try doing the join and then scale back up. That can work because when you scale the model down the gap between the edges also scales down as well and that can then put the gap below the join threshold.

To scale all your objects by one tenth in size, select them all and then run the Transform > Scale command. At the first prompt for the scale origin type in 0 and push enter to use the 0,0,0 origin point as the center of the scale, then at the next prompt type in 0.1 for the scale factor and push enter. Then after you do the join repeat that but use 10 as the scale factor to reverse it.

If a surface has some kind of slightly odd or mangled edge structure which is preventing it from being joined then sometimes it can help to untrim and retrim the surface to repair it. There are some notes on how untrim works here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=444.4
and also there's a longer tutorial going over object repair techniques here:
http://moi3d.com/forum/index.php?webtag=MOI&msg=446.17

- Michael