FL is one of the guys in a group of guys

Reply series is very specific in this aspect but here are some dramas that (at least partially) fulfill your request. Usually, there is not such big ambiguity between which male lead will FL end up with. She loves first male lead  and is friends with second male lead but both love her. Also, Reply dramas are ridiculously high quality while harem dramas usually are not that good and are more fan-servicey

Also, I think that harem dramas might be something you are looking for in general?

Anyways, here are my recs (i underlined dramas that are very good and i rec them in general):

Sassy Go Go - very underrated cute school drama. FL and both MLs start as enemies but become soon members of big team (i rec this drama no matter if it fulfills your requirement)

Accidentally In Love - there is only FL, 1ML and 2ML in the group of friends and FL always likes only 1ML

Atashinchi no Danshi - (i dont remember much about the story lol. but i know it is harem drama)

Boys Over Flowers/ Hana Yori Dango/ Meteor Shower (or any other iteration of this story lol)

Flower Boy Ramyun Shop - not my favourite drama but might have the themes you are looking for

Flower Boy Next Door - not my favourite drama but might have the themes you are looking for

Moon Lovers: Scarlet Heart Ryeo - historical drama, mix of cute and political/tragedy. big budget and very good in general

You're Beautiful

To the Beautiful You/Hana Kimi

+1 for:

• Sassy Go Go - the wider group involves two school clubs (a dance club made of bad students and a studying club made of top students) that are forced to merge, and the smaller group involves the ML and SML, who are best friends, which becomes a trio when they befriend the FL (her best friend also happens to be male but he doesn't really join the trio)
• You're Beautiful - the FL pretends to be her twin brother and joins a male idol group
• To the Beautiful You - the FL pretends to be a boy to attend an all-boys school in order to support her idol, a student athlete (high jump) who is currently struggling - the "group" dynamic is a bit looser in this one
• Coffee Prince - the FL is mistaken for a boy and offered a job at a coffee shop where the owner and staff are all good-looking guys (the "group" includes the owner, the owner's best friend, and the other staff members)
• Moon Lovers - the FL befriends most of the princes in the royal palace

I'll add:

• Sungkyunkwan Scandal - the FL uses her sick brother's identity to get work to support her family, but she ends up becoming a scholar at Sunkyunkwan and due to various competitions and circumstances she and three other male scholars become friends known as the "Jalgeum Quartet"
  
• Thirty But Seventeen - through a series of events, the FL ends up living with the ML and SML and becomes their friend / part of their family (in a wider sense the "group" includes the SML's best friends / rowing team mates)
  
• Dream High - the FL, ML, and SML are all part of a small special class at a high school for the performing arts (e.g. singers, dancers, etc.) - the "group" dynamic is a bit looser in this one but it definitely has the romantic dynamic you're looking for

Of the above, Coffee Prince, Moon Lovers, and Dream High are the ones where the FL has clear romantic feelings for the SML at some point before choosing the ML. 

Others worth mentioning that don't quite fit your criteria:

• Love O2O - the FML and ML are both computer science majors and top-ranked players in an online game; they get "married" in the game and the FML joins the ML's team which includes his friends / roommates and later meets them all / becomes their friend in real life
• Mr. Sunshine - the FL is secretly part of a resistance group, which includes a limited number of core members, but the ML, SML, TML and others all assist with their efforts at various points