Documentation of various linux programs often states that after adding user to some unix group you need to re-login in order to use new privileges. Although this is definitely the easiest option, it is still possible to enhance user's groupset without re-logging.

After adding user to the group you can use newgrp group_name. This is an old tool (from the time when unix users could use privileges of only one group at a time) that changes the current group for user. Additionally, it tries to add the group to the current groupset for the user.

For example:

$ id
uid=1000(pawel) gid=1000(pawel)
grupy=6(disk),10(wheel),11(floppy),18(audio),
19(cdrom),27(video),80(cdrw),85(usb),100(users),
443(plugdev),1000(pawel),1004(www),1005(ssmtp),
1006(vboxusers),1007(kvm)

$ newgrp wireshark
$ id
uid=1000(pawel) gid=1008(wireshark)
grupy=6(disk),10(wheel),11(floppy),18(audio),
19(cdrom),27(video),80(cdrw),85(usb),100(users),
443(plugdev),1000(pawel),1004(www),1005(ssmtp),
1006(vboxusers),1007(kvm),1008(wireshark)

Unfortunately, the current group changed. To have it back to user's default group just run newgrp again:

$ newgrp
$ id
uid=1000(pawel) gid=1000(pawel)
grupy=6(disk),10(wheel),11(floppy),18(audio),
19(cdrom),27(video),80(cdrw),85(usb),100(users),
443(plugdev),1000(pawel),1004(www),1005(ssmtp),
1006(vboxusers),1007(kvm),1008(wireshark)

This only affects the current shell, but should be enough to just run a single program (like wireshark or some cd burninig software like k3b).

Oh, and just plain su - mylogin would also work - but it asks for a password :)