If your theater group is mounting a production of a musical or play, then you can create T-shirts for the show. Many shows have logos or theatrical artwork associated with the play. You can use this artwork as an image, with the name of the show in the same font as it appears on the script or musical book. Beneath the graphics and name of the show you can also add the dates the show will be held. When you and other members of your theater group wear the shirts around town, it will serve as advertising for your upcoming show.
If you belong to a sketch or theater group that meets to rehearse plays and other theatrical material, you can create T-shirts that name generic characters. For example, one shirt might read "Dad" or "Mom." Other shirts might read "Soldier" or "Businessman." Then when you are acting out scenes with these characters, you and members of your theater group can wear the shirt that corresponds with these characters. When not in use, they can be kept in the prop box. Every so often, assign someone to wash them and return them at the next theater gathering.
If you belong to a particular theatrical group, you can create a T-shirt for that group. For example, if you belong to the "Springfield Community Players," the T-shirt can have that on the front of the shirt with an associated graphic. If your group already has artwork it uses to represent the group, then you can use that artwork. If not, you can look for generic theatrical artwork, such as the drama and comedy masks. You might even add the year you were established on the T-shirt, or the productions you presented on the back of the shirt.
Some drama clubs and community theaters do not have the money to rent costumes for their production. Therefore, they might decide to dress in very plain attire as a choice when putting up a bare bones production. You can order short- or long-sleeved shirts that are simply black, white or some other solid color you desire. This can be effective if you are putting on a dinner theater performance full of many different skits where people are playing different characters. It can also be effective if you are performing things such as staged readings. This will give your group a uniform and professional look.