It is possible to make an eiruv techumin for a period of many Shabbatot.[3] (To do so, one must use something with a long shelf life and ensure that it is in a safe but accessible place.) When making the declaration that accompanies the placing of the eiruv,[4] one should indicate that it should take effect only on the Shabbatot when he might want to use it (see more on this below).[5] This could be important for the following reason. An eiruv techumin does not fundamentally increase the distance one may walk. Rather, it changes – hopefully in the most advantageous way – the central point around which the circle with a radius of 2,000 amot is calculated. On a week that is not your turn to lain, you might want the freedom to walk in a different direction, which the eiruv could actually preclude.
The same food that was put aside for that purpose for one week can be reused. You do not even need to know before a given Shabbat whether you are going to activate it that Shabbat; you can rely on the original global declaration.[6] This is because we maintain that regarding Rabbinic halachot such as this,[7] certain details of a halachic process can be determined retroactively (bereira).[8] Therefore, the eiruv is operative based on the original declaration, and the days for which the declaration will apply can be decided later.[9]
The next question is if everyone in your group can share an eiruv. The Shulchan Aruch[10] says that one person can place an eiruv techumin on behalf of a group and make a kinyan for its members. This works even if it is unclear at the time that the eiruv is laid down who will be included for a given Shabbat. (For example, the mishna[11] talks about all those who will go to a house of mourning.)
There are, nevertheless, a few conditions that must be satisfied. The members of the group must be aware of their possible inclusion in the eiruv before the given Shabbat begins,[12] even though they do not have to decide at that point whether they want to be included for that Shabbat.[13] There also must be a requisite portion of the eiruv for each person who needs to be included for a given Shabbat.[14] Since the requisite amount is the quantity of a food that is sufficient for two meals,[15] this may be challenging in cases in which it is needed for a potentially large group over time. Although you are talking about a small group, we will discuss some of the options.[16]
One technique is to use a food that is consumed in small quantities. Unlike an eiruv chatzerot, which must be of bread, an eiruv techumin may consist of anything edible,[17] as long as there is an amount that would be used in two standard meals.[18] For drinks, this is two revi’iot.[19] Regarding foods used as relish to put on other foods, the amount can be even less,[20] such that a bottle of something of that sort could be effective for an entire group of people over time.
Another system is that each week, after using the eiruv, the person who used it makes a kinyan to pass it on to the next person or back to a central person who is in charge of making a kinyan on behalf of the relevant participants. (The easiest is a kinyan sudar, in which the transferred object does not have to be present.) According to the Shevet HaLevi,[21] it is not even necessary to make a kinyan to pass the eiruv back, as the user of the eiruv each week may acquire it conditionally for only one Shabbat at a time. Using this method, it is sufficient for the eiruv techumin to consist of a quantity that is enough for the maximum number of people who will use it on any given Shabbat.[22]