In BIDS, labels need to be alphanumeric, so you’re not constrained to designators like _0 and _1 to indicate group. Also, underscores (_) are not valid alphanumeric characters, and have a semantic significance in BIDS filenames. You have a couple options.
Put the group name right in the participant label, e.g., sub-control01, sub-treatment01, … In general, though, you shouldn’t count on software that you use to distinguish groups by patterns in the labels; those are there for human readability. You should specify the group in participants.tsv, as well.
Simply assign labels and then use participants.tsv to distinguish the groups.