The following provides one solution to the problem. However, it seems unnecessarily complicated to me. I would appreciate if fellow Stata users could suggest a simplification.
The dataset is the following:
ID communityID treat
1 1 1
2 1 1
3 1 0
4 1 1
5 2 0
6 2 1
Where I count the frequencies of (positive) treatment per group. I add these frequencies (treat==1) as a new variable (which is equal within each communityID).

preserve
contract communityID treat, zero nomiss
drop if treat==0 // IN ORDER TO HAVE m:1 MERGE
save treat.dta, replace
restore
merge m:1 communityID using treat.dta
The resulting data NRPS.dta then contains the collapsed information
communityID treat _freq
1 1 3
1 0 1
2 1 1
2 0 1
What is the best way of solving this problem without saving and merging data?