Strange output

[buran]

line 8 is the root of your problem
g[3] is a string and you iterate over string. so u is each char in that string. then you try to split single char at ',' (line 9)

what i suggest:

def get_group_users():
    f = open('group.example', 'r')          #f is file
 
    users = []                              #obvious
    for l in f:                                     
        g = l.strip().split(':')                    #g is group
        if (g[0] == "users"):
            users.extend(g[3].split(','))
    return users

print(get_group_users())

Output:
['kintaro', 'john', 'autossh', 'test']

Now, what I would do, to make better code

def get_group_users(file_name):
    users = []
    with open(file_name, 'r') as f:
        for line in f:
            if line.startswith('users'):
                *_, group_users = line.strip().split(':')
                users.extend(group_users.split(','))
    return users

print(get_group_users('group.example'))

Output:
['kintaro', 'john', 'autossh', 'test']