Heyo!

quick question. working on a big scale mod, and trying to get my friends involved. Wondering if it's possible for me to work on the exterior cell, and they on interiors, and later merging these into one file. Is that an easy thing, or can things get messed up easily?

Things can get messed up when merging, yeah, You can use the CK's version control or, when TES5Edit is publicly available, you can use it to carefully merge any amount of stuff.

Hmm.. so is it best to give him a copy of my .esp and let him go crazy with new cells within that and then merge them? or let him use a seperate .esp file then merge those?

Firstly you'd need to convert your main .esp into an .esm, and then use Version Control to merge any .esp files into the main .esm.