Based on #4533 (comment)

DynamicUser=yes
User=hola
SupplementaryGroups=1

systemd[1]: a.service: User lookup succeeded: uid=1001 gid=1001
...
sh[184]: uid=1001(hola) gid=1001(hola) groups=1001(hola)
...

#4533 (comment)

And I have the suspicion that we should use the same codepaths for this regardless if DynamicUsers= is set or not. After all, DynamicUsers= means "use a dynamic user if there is no static user for this". It does not mean "use a dynamic user, and fail if there is a static user"