I will give you the example of the one I have and try to explain it the best way I know how. I had problems with this for weeks until I finally understood it.
eval(eval(SharePointListItem_RW[(reportsto = Emp_Pos_Nbr) and (emplid != "")], 'concat("POS#-", d:posnbr, "_EID#-", d:emplid, "-", ";")'), "..")
eval(eval(Repeating Group (not the field you want) [Filtered item], (Concat only uses the ' and not " ) 'concat(notice here it is d: or m: this coincides with the repeating group you are using. If the repeating group is in your form you will use a my:fieldnameinthegroup and if it is from a datasource you use a d:fieldnameinthegroup)'), "..")
As you can see I have two of the fields in the row where the filter has selected from the repeating group, and seperating them with a string: POS#-FIELD_POSNBR_EID#-FIELD_EMPLID-; Repeat for next row
Hope this helps, if you have problems post your string
Barry