In a formula in a cell:
=personal.xls!myfunct(a1:b99)

If you saved your personal workbook as an addin (personal.xla), then you can
refer to the functions in that addin just like they were built into excel:
=myfunct(a1:b99)

If you want to access the functions in your other project (via code), you have a
couple of choices.

You could add a reference in the "receiving" project that points at the
"sending" project.

First, it's probably better to give the "sending" project a nice unique name.
Open the VBE
Hit ctrl-r to see the project explorer
select your workbook's project.  
It should look something like:  VBAProject (book1.xls)
Hit F4 to see the properties window
Type the new name in the (Name) property

Save this workbook.

Select the "receiving" project
Tools|References
Look for that nice unique name and select it (check it).

Then you can use the functions in that other workbook just like they're built
into VBA.

Dim Resp as long
resp = myFunct(activesheet.range("a1:A7")

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The second way to have your code call a function in another workbook's project
is to use Application.run:

application.run "'FullfileName.xls'!macroname", myargument
or to return something:
resp = application.run("'fullfilename.xls"'!macroname, myargument)