As in the example you mentioned, call plt.scatter for each group:
import numpy as np
from matplotlib import pyplot as plt
scatter_x = np.array([1,2,3,4,5])
scatter_y = np.array([5,4,3,2,1])
group = np.array([1,3,2,1,3])
cdict = {1: 'red', 2: 'blue', 3: 'green'}
fig, ax = plt.subplots()
for g in np.unique(group):
ix = np.where(group == g)
ax.scatter(scatter_x[ix], scatter_y[ix], c = cdict[g], label = g, s = 100)
ax.legend()
plt.show()
